Power rail faults are found with Strip Down testing.           
First desktops then laptops, never fake Desktops.

You are here for 1 reason, and that is you suspect the power rails (all feeds of power) are overloaded with shorted modules of any kind in the PC.
The PC is dead, or turns on for 1 second and then turns it self off and all fans go dead.
The first thing to do is remove all USB devices, and then unplug the HDD (all of them) an the optical drive, (CD/DVD/BR- d) That is correct all these can short out.
USB is not really much consern here, but some lame old cheap PCs lacked proper USB power feed current limits back in the year 2000. (modern USB has electronic fuses that make USB safe)
Most Desktops are super easy to Diagnose in this  ordered list: I will do Desktops then Laptops last, for most tablets have very little anyone  can do, but replace the old dead large shorted battery.
Keep in mind there are 4 stuctures that can be shorted, and make a PC dead, power, data, address and controls. (nut just power rails)
  • Do the long power reset first. (all power an batteries all  removed ) as seen here.
  • Remove all USB port devices next. ( most times this will not cure 99% of Dead PC's, only relics.  (no need to remove the VGA/DVI/HDMI monitor as they can not short out PCs power rails.
  • Open the left side case cover, an remove all HDD and the optical drives (CD or DVD etc) (PC turns  on now, bingo for shorted out drives.
  • The Motherboard has only a few things lift now, that can short, but next is PCI-express cards are remove 1 by 1 to see if say that Graphic card is shorted.
  • Next up is RAM, DDR RAM.  Remove them all (most PCs have at least 2) and test them one by one in Bank 0 slot (your manual on your PC shows how to use one RAM stick only, read this please)
  • The CPU in almost all cases never fails, (it has huge internal protection) but only lightening hits on the power or other cables (LAN?) can blow it up. It most not have dead fan or missing heatsink.
  • My Motherboard had M.2 (tiny SSD cards) that can short so out those come to. (high end PCs have these)
  • If  you get this far and PC is still dead the PSU is bad,  or needs bench testing, hit the home button above, to see how.

Special case #1:

Some PCs with dirt cheap and very old  PSU, can stay turned on, but PC is dead , like a dead 3.3vdc rail dead. (and has no under voltage protection on that 1 pin)
DC RAIL TESTING IS EASY on  Desktop: (if you own a $20 DMM meter)
  1. Lets say the PSU fan now spin ad blow air, next is to test all power rails with the DMM meter.
  2. The voltmeter is set to the 20vdc range, and connected black lead to ground.(COM, or common black wires , pin 24 is easy to reach for ground to DMM lead)
  3. The red lead of DMM meter touched 1 by 1 to each supply output pin rail as seen here.
  4. If  voltage tests  fail, then the PSU is bad.  (or that rail line is shorted)  
  5. The motherboard can be bad, the MOBO has some low voltage (1.8v) regulators present (VRM's) that can go bad and the MOBO is smart enough to detect that and then sets power on to OFF.
  6. PSU bad yes, then  buy a better PSU
The wiki spec on ATX2 shows the maximum errors allowed for voltage  The wire colors are standard on ATX spec. Desktops.

A new PSU rated at 500watts or more and v2.3 version up.. $39 at newegg.com "Thermaltake (tm)"
If still dead, the mobo is history.(I've even removed the CPU last step, but I have never seen any CPU short power yet, but can have bent pins due to bad service)  Or Lightenting hits sure.
Keep in mind the fans will be dead if BIOS is dead or CPU missing.  This is worst case, a full strip down and build up from a minimum configuration.

Warning ( not too mobo made have the same pin out of the below connections, (unless they are axactly the same) there is no ATX spec on this pins, like most the others have.
Before working any mobo and the power switch cable is missing or too far away learn which pin pair is power on. and jump it momentarily.

 end desktops (real)

LAPTOP (LT) power structures are complex.  (the actual PSU is hidden deep deep inside all LT) 
Here is a page I made on Laptop power isolation testing.

The laptop can have many modules to pull first to clear any short. (even useless now, dialup modems tiny card can short)
This is all about power rail (or data/address/control ) structures, shorted, by doing fault isolation testing.
In man cases(no pun) the cases must be totally disassembled to do the work.
The fake Desktops can be diagnosed just like laptops. (skip the huge battery topic)

version 1.  12-12-2018