Black screen but backlight is on

Hi,
my grandma’s phone suddenly broke, as long as I know she just unplugged it from the charger.

The screen is blank, the backlight works fine but there is no image (no logo, no recovery screen). If I turn on the phone normally it start vibrating endlessly.
If I try to turn it on while charging it does not vibrate but the screen is still blank.

I also disassembled the phone to see if there were any visible issues, but there aren’t. I removed the screen and tried to loosen the flex cables and check them out, unfortunately the problem is still the same.

Do I have to just replace the screen, or it can also be a motherboard issue?

Thank you for your help.