Thanks so much guys for the quick replies! there have been updates:
I reject the sensor hypothesis. I'll explain how it went: I tried to raise the brightness to maximum and the screen turned off again. So I tried again after the forced reboot, and the same thing happened. I then left the iPad on for a few hours and everything went well, without touch the brightness.
I tried turning it on again now and I can't even force restart it anymore, but it still stays on with blank screen. I'm guessing it's a backlighting problem.
do you have any advice? I can't find anything online for this specific model