The PC is dead (means BIOS pages all dead)
"Strip Down" testing? or what is a Minimum system?
A.k.a. Remove your shorts!
On desktop First make sure your monitor is not dead, and works on any other PC.
See PC flow chart tests here.
Do not fail to see of P.O.S.T errors exist, via beeps or flashing LED codes !
For PC with dead fans and no splash, that is a true totally DEAD PC. (read on)

First off we must  know what a computer is and now it works.
The PC is a complex extension of the  von Neumann architecture, or
Princeton architecture.
you must learn is that the computer or PC, (personal computer) has vast internal (or external) shared resources.
This means if one module in the PC even the battery shorts, the PC goes totally dead, that means BIOS goes dead too. (if the PC can not compute at all BIOS is dead) or at 5 years old the RTC is dead.
The word used is bus, and  rail if any bus in the PC is shorted out, or cut open (damaged) the PC goes dead,  rails are DC power, 1.8, 3.3,5 and ,12vdc power rails are more.   They all have regulators that shut down for any SHORT.
There are buses called DATA , ADDRESS and  the control bus, if those fail the PC is now dead.
The first matters in  diagnosis, are making sure all bad or shorted batteries inside any PC are removed first then replaced, in the case of laptops, you can remove the huge battery and run it forever like that on the AC power pack.
The PSU , power supply must not be dead.(bad in any way) we bench test them all. (under bench load banks , a tool savvy techs have)
All PSUs and Regulators when overloaded simply turn off, this is to prevent that nasty guy called FIRE and SMOKE from happening, by law and clear design.
All rail voltage regulators in the PC do the same thing, they shut down at any power overload (or shorts) shorts means in tech talk a short circuit or gross overload .  (factoid #1 PCs shut down when too hot overheated)
Once we are sure all batteries replaced , tested or removed, we test the PC for other shorts.
Many old folks want to ask "where is the FUSE here", the answer is there may  only be 1 fuse or none and is seen here on a 65watt laptop power pack sealed in plastic not accessable at all.(short of cracking case open with  a hammer)
A PSU will have  4 to 6 amps fuse on the HOT AC line or a 5ohm special resistor that blows up overloaded. (or some Poly fuse and no spec. at all told)  If blown all voltages (rails) out of PSU are dead. (new PSU time)
Most modern PCs have no fuse on any mother board, (mobo) that is because the regulators do that job  now,  just like magic, The old desktop ATX PSU may a have 1 fuse.
All PSU can be bench tested, a real ATX can be tested with 1 paper-clip jumper here !.   The maker HP uses very strange custom non  ATX standard PSU, with these odd rules here and seen here.
Do not touch the insides of any AC power supply or you can get a 375 voltage peak,and you get a shock and be elecrocuted dead. (that can be lethal , ok?) (simply stay out of the PSU dog house, stay alive)
Touching the mobo is safe do not open up any kind of PSU like I do. I'm HV certified tech. (chicken stick in my tool box and more)
Finding shorts  can be easy or hard. (luck and fate win)
Batteries in PC are shorted, not just dead and bad but shorted. ( laptops  old love to catch fire shorted huge battery , ask Sony how that works for them. See recalls here, and fire)
USB port cables or devices plugged into the PC can be shorted, so unplug them. (first)
Even a desktop HDMI or DP video cable can short, unplug it. (this pin here is the hotswap power  pin to any monitor or TV.) A bad cable here can short.
To test the All AC  power , unplug the PSU AND BOTH ENDS OF ANY POWER PACK, for 1 hour, then connect AC cord and push power "ON" button for 10 seconds and release it.
PC still dead here.

Fans run powered on. if not fix this cause first. The fans must be seen to spin or heard or felt the air blow out.  Look carefully is best.  (desktops have 2 to 6 fans, latter is gaming PCs huge, CPU and PSU are first to spin.)
Splash screens BIOS puts to the screen the makers logo called SPLASH,  if not the screen is dead or PC is dead.
BIOS hot keys all work, like dells F12, for sure.
My Dells have  BIOS log page, inside BIOS , if POST fails this log shows why, love Dells we do.
No post errors. if you get beep codes or flashing LED codes, your OEM manual covers all that, read it?
My Dells also have a migic black power panel that has 4 LED behind it hidden until they glow, and this 4 bit code is a POST error. (black = good) not blank = failed and why.
If still dead, try the strip down testing.
The cheaper the PC the worse this gets,  no books worth snot, no built in diagnostics like Dells F12 hot key.  You do get what you paid for.

What we do as techs, is remove all parts, 1 by 1, from easy to harder to do .
We land on just a power pack (PSU) and one motherboard (MoBo) and 1 stick of ram.
This is what is called a miniumum system.
Rule #1 is do not hot swap any parts inside the PC.  Do pull the AC power cord from the wall and then press the power botton on PC 3times or more to discharge all capacitors inside the PC.
On laptops the AC pack pulled and DC battery huge left out and not connected at all.

What is a minimum system?   A.k.a Strip down testing.  DESKTOP first.  ( I have  Gaming PC (DIY) that is vastly more complex the below is just attic junk box fodder)

See my jumper wire on the power button jack (button missing to prove I don't need any button)
Here is my old junky desktop from the attic junk box.
This is a demo, of what to do on a desktop, done in a way to show gee mom, no case needed at all to test any PC. (if you know how)
No GPU card needed in 99% of cases, for sure  none needed with i3 up Core CPU. with built in iGPU, and you see a  VGA or DVI port on rear of mobo.
Remove all PCI cards (yes they can short)
All USB gone. Some classes of USB power wire shorts will shut down some cheap made PCs (lacking real smart USB Vbus chips)
All RAM but one stick test all sticks 1 by 1, just in case one is bad.
Leave CPU alone, if its pins are not bent it is not bad.
The first test is if the fans spin, if not the PC is dead.
The next level after fans spin is BIOS screens work.
I connect up the VGA display and see the HP or Dell splash logo. If not the mobo is dead (we bench tested the PSU is known good, and mine is a spare PSU in a box, marked  GOOD)
The coin cell is new. (if coin cell is  dead the BIOS goes dead or just nuts.)RTC  Coin cell dead cause the NVRAM to lose all data.

Warning:  No two mobo made have the same  power ON JACK  pin out for the below connections front panel , there is no ATX spec standards  on these pins !  (READ YOUR PC MANUAL) RTM .
Many OEM mobo makers do not document pins here at all, but ASUS and Gigabyte do, a top DIY mobo sure does for decades now , like Z270 prime or z370 . (my 2 favorites)
These 2 power on pins are marked as , 1 is ground other is "power on" pin logic input, and is easy to jumper (momentary)
The power panel or power button  are missing here.
(it is all low voltage here  , all things on the mobo are safe voltage wise.
(OEM PCs will set errors to the screen if some features are missing now,(I ignore them) like case fans missing  , front panel missing, intrusion alarm switch missing. (thermal sensor missing case temps)

Find the 2 pins and  jump them momentarily, one pin is ground pin , other is power on INPUT pin.

End desktop.
If confused guess, replace something , anything, guessing can work if thick wallet.

LAPTOP (LT) power structures are complex.    This PC runs W10-64 .   With some work.
This is worst case example (good no?) and is not some lame cheap,  1 circuit  board  TABLET PC. nothing unplugs but LCD)
This is a top brand Laptop.
The power jack at power pack is 19vdc , of not the pack is bad or the mobo is shorted out.
It has over 23 models to remove.
The RTC coin cell is fresh. (aka CMOS coin)
The battery huge was remove first and the 19vdc power pack tests good on my bench under load. (10ohm load in my load bank)
I remove moduels 1 by 1, from easy to hard, RAM is under a hatch and so very easy  to remove.
USB was first. removed all USB devices.
The modem card is thrown in the trash, I don't need dial up ever again, out it goes.
Wifi card can short out and it is pulled next.
Then all sticks or RAM, DDR3 here  and tested 1 by 1. (this PC has 4 slots DDR3)
I remove the USB3/VGA card and then the express card (the latter is an option)
The VGA port works even with screen 15 jack pulled to LCD panel.
This rare PC has  GPU card, and only a spare can you test this PC.

Last actual LT testing (laptop)  NO BATTERY HUGE ALLOWED here.
Test on AC pack only. HP 8540w Elitebook,  Look ma' no case !  The VGA port does the same thing even with no factory LCD plugged in. I have the mobo sitting on random plastic things.  Crud  but effective.

To work on any laptops, learn not to damage the frail ribbon connector jacks, (there are 3 kinds, and 2 are super fragile, the hinge type is worse and wedge type near worse. (the simple Push/on/off type are rugged.)

The fact is monitors tell you the PC is dead with this screen warning. (no signal)  We can test the monitor on a 2nd good working PC, easy, never assume it is good test the monitor.
PC dead or even theVGA cable bad or its pins bent?
#1 cause of dead mobo is bent pins. ( CPU pulled or buying used bad MOBO) #1 cause.

Rule 2? never buy USED mobo off fleabay with the protective CPU socket cover missing or find out later the  packing material wrecks these pins in 1 second flat. DOOMED.
Do not buy any cards or mobo with VIA, Nforce or SIS chips of any kind they are are not supported now. (the end has come for those chips)
If confused or fear 35 laptops screws, get the PC repaired in a real shop, that has no such fear.

Citation 1 , NVRAM +  RTC coin cell battery life span spec.'s, RTC means real time clock chip,  an actual wrist watch chip is inside all PCs made 1981 to now) 5-7 year life spans !

C2 Li-Po lithium battery life and run time rules.  Abused can fail in 1 short year, (overcharged or under charged)

version 5.   leaner.