Laptop screen failures, all.         

Laptop  "LT", only screens.  ( lots more to fail here, due to more parts in play and shared power !)   This is not a monitor it's only a SCREEN now.
The laptop has no OSD brain (like desktop monitors above have but only the OS BRAIN CPU (Intel i7), can control the GPU chip and screen now.)
The i7 processor and BIOS runs the GPU and all BIOS screens work , even with no OS or even any HDD at all installed. The #1 goal is to get BIOS screens working first!!!
The laptop we first check that  we can get to the BIOS pages (on mine it's the ESC key,. older HP computers used F10 key, and is prompted !)
Check the causes of DIM screen first.  (remember if the LT thinks the main battery is weak it dims the screen ,or  if battery is bad confusing it.)
  1. Connect the line power pack 19vdc 65watts or more to the wall power and Laptop connector.
  2. Remove  big battery that snaps into the bottom,  (if a tiny cracker thin notebook , with no easy out battery, just remove the bottom of the case and then unplug the battery inside) and test the screen this way.
  3. Turn on the PC laptop, if you see text now then know that   the screen is not dead.  (see OS dead) {operating system}
  4. As you power on the HP, hammer the ESC key at power on actions BIOS screens always pop (if the RTC CMOS coin cell battery is dead, BIOS can be dead)
  5. The BIOS  screens then give you text menu's  and options,  if that works, the screen is NOT DEAD,  but the OS may be dead or a dead HDD.)
  6. If the screen is still dead,  no HP logo shows or the words, "hit ESC key to enter setup" never shows up do the CCFL dead?  flash light trick next.
  7. To further prove the screen is dead connect a Desktop screen to the LT read VGA jack. (if HDMI use that to a TV) if this works ok, the local screen is dead (Fn+F4 is the hot key to activate the VGA jack)
  8. More examples of black screens (but screen it self not bad) are covered here.  black-screens of death
  9. Windows drivers wrong, bad, corrupted , missing or infected, down load them fresh from the OEM maker site and use them.
In most cases,  if the screen never shows, any text , any logo's, any BIOS prompts, is a  bad screen or bad CCFL tubes linked above  or  inverter card.
Or BIG Battery shorted or a bad power pack.
The big test of all is grab any desktop monitor and plug it in to the rear of your laptop and test this port for good video.  (FN+F4 key on mine toggles, internal/external/both)

If the screen is dead,  and the flash light tests fails.
That is a dead PC, just about all the below cancase this failure.
  • Shorted main huge battery (remove it)
  • RTC coin cell can cause the BIOS to fail hard and not even POST. Replace it  every 7 years (so says the Pink Bunny (Datasheet facts)
  • The Power pack AC can fail and  not reading 19vdc with a DMM meter, never below 18.5vdc and for SURE LESS voltage we test it at the mother board jack.
  • 2008 PCs had bad GPU chips.
  • Any module seen below that shorts out inside the PC can cause a DEAD PC. (we strip PCs down to find who is shorted)

HP8540W (Elitebook) With comments. The USB3 ports are dead in BIOS, so no booting from there. Ever. Now can W10 use the HP security packages, but that I do not need.
This is my Laptop block diagram, the screen shown, at the large red arrow. This is LED backlighted screen and very reliable.
The video card below  card looks like this, in the below Laptop. This PC can be had used for $100 (a steal)

Mine now has 2 SSD drives with DVD removed.  It screams fast, and is RUGGED ,un like most crazy priced $1200 Laptops seen now. (I call them cracker PCs as thin as a ...)
Some new cracker thin PC's are super  frail, LT and  need this part to do the external monitor tests. (lacking VGA/DVI port jacks at all)
Warning some notebooks the battery inside is super hard to remove not just simple screws,but glue together in horrid sandwich. Like this.
 Before condemning any LAPTOP /NOTEBOOK remove the huge battery first.

version 1.  12-9-2017