EDIT: I managed to solve this issue, and it was one of the most curious troubleshooting I ever had to do. TL;DR: A magnet (for the hall sensor) was interfering with the screen's inverter board.
I have an old HP G61 which I left for my older kid to learn some coding. Just yesterday he was using the PC in the morning without any issues. When he returned later afternoon, the PC was acting weird.
Soon after booting/rebooting, the screen would go black after approximately 5 seconds. This isn't a Windows issue (or any other OS for that matter). If I go to the BIOS menu, or boot device selection, or if I boot onto another OS from a thumb drive, the same issue would happen.
The screen might be black but the PC is functional. CAPS/NUM LOCKs can be switched on/off, and PC would make sounds when certain keys are pressed. I can also see the drive read/write activity LED blinking as Windows boots up.
Curiously, if I perform a warm reboot via CTRL+ALT+DEL, the screen would come back during the reboot, but again after a few seconds it would go black.
If I connected the laptop to an external screen via HDMI it works just fine. Curiously, every time I change the screen projection on Windows (duplicate, extend, etc) the screen on the laptop would again show for a few seconds, and then it would go black.
At this time I ruled out the issue being the GPU, and I thought it could be either the screen or the screen cables.
I opened the laptop and gave it a good clean with IPA alcohol, added new thermal paste, cleaned the fan, etc. But still I couldn't fix the issue.
After some research on why the screen might be very dark, I read some notes on inverter boards for LCD screens. Seems like what I was experiencing has to do with voltage drops in the inverter board. I started to look for a replacement and on eBay and they seem affordable. So I decided to open the screen this time to get the exact bardcode/model for the inverter board.
Now for the plot twist: I was surprised to see a small MAGNET stuck on top of the inverter board, right on its coils! I removed the magnet and BAM! the screen was now working. I approached the magnet to the coils again and I could see the screen flickering and going dark.
So where does the magnet came from? It is actually from the screen frame, and it is used to activate the S3 mode (standby) when the screen is closed to aprox 35 degrees. Without this magnet, closing the screen does not activate the stand by mode. I placed the magnet back in its slot located at the bottom-middle of the screen frame and I glued it with some gorilla glue. Closed everything and now the laptop is working again.
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