I had the same problem. While rebooting the laptop the screen would be on for a few seconds after it finished rebooting then the display would go black. If you looked very carefully at the screen you could see the image was there but there was no light behind the screen to make the image visible.
I removed the black plastic display bezel cover that goes around the edge of the display screen to check out the lights and wiring. I found a small magnet that belongs inside the display bezel had fallen out of it's slot in the plastic bezel and was stuck to the inverter pcb (printed cicuit board). To fix this problem I secured the magnet into it's slot inside the display bezel with a bit of glue. After fixing the problem and putting the display bezel back together the laptop worked normally.
This magnet is how the laptop knows the lid is closed. When the laptop is closed the magnet is moved closer to a magnetic switch on the inverter pcb and activates the switch. The laptop thinks it is closed and shuts off the lights behind the display screen. When the magnet fell out of it's slot and stuck to the inverter it activated the magnetic switch all the time whether the laptop was open or closed. This caused the laptop to think it was closed and shut off the lights behind the dispaly screen as soon as the reboot finished.
Try to press on during five seconds.
JMG
SOURCE: HP Laptop won't come back on after sleep mode - have to reboot
Check the Power settings in BIOS. Some of the laptops have a S1 and S3 setting. Check to see what setting it is on, and switch to the other one. Most likely, it is set to S1.
Without the model number, it is kind of hard to tell you how to navigate through the BIOS, but with HP laptops, you can get to the BIOS menu by pressing "F10" when you first turn the computer on and you see the HP logo.
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