What to do with a black screen?

drpeppercan said:

Laptop: Lenovo IdeaPad Flex 5, Aug 2020
Even the F2 won't work to access the BIOS, nor the F12 won't work to bring up the EFI menu. I also tried using a Live USB stick with rEFInd - a booting manager - but that didn't work either.

What else can I try?

Hook another monitor or TV to the laptop, see if that works.

Check the ribbon cable going to the screen (internal).
Re-seat the RAM.

When you turn on the laptop if there are no lights or fans spinning, then the laptop is not powered up, bad battery, or wrong power adapter plugged in, or the motherboard has failed.
If the fans are spinning etc, then as stated try an external monitor or TV

drpeppercan said:

I plugged a monitor with an HDMI cable but the black lit up screen continued showing on the laptop, not the monitor. It's not a 100% plain black. The screen seems to work, except that it's mostly showing a black hue. Still, I don't know why the monitor wouldn't show it. Isn't it just a matter of plugging the HDMI cable from the laptop to the monitor?
The keyboard did lit up, as well the tiny light on the power button. I don't think I heard fans.
I guess I'll find out how to open it to check the ribbon cable as you advised.

When you hooked up the other monitor, did you get an image of any kind?
Also... on the "other" monitor you hook to the laptop, make sure you set that monitor's input for HDMI, or w/e cable you used.

Let's say you hooked up a 2nd monitor properly, and that the cable you used is good.
Then you should be able to get into the BIOS on the 2nd monitor.

If you can't... then there is something wrong with w/e "makes" the image.
That could be the grfx card, or CPU, or RAM that needs re-seated, or a bad motherboard, etc.

If you can get into the BIOS on the 2nd monitor, then that indicates that there is a problem with the laptop screen or the ribbon cable that supplies the image to the laptop screen.

Make sure you are using the correct Power Adapter for the laptop and that it is plugged directly into the wall.
There is no need to reseat the ribbon cable to the laptop monitor if the secondary monitor doesn't work.
If you don't hear fans spinning then that is a sign of a hardware malfunction. The GPU Graphics "card" is soldered onto the motherboard, so if nothing works, chances are great the motherboard has failed.
If you are brave and want to open it up and reseat the RAM, here is a video how
Lenovo Ideapad Flex 5 14ARE05 81X2 Disassembly SSD Hard Drive Upgrade Battery Replacement Repair - YouTube