Had a strange thing happening to me. My screen turned black after opening Signal app. Thought the device crashed.
After reboot even the lockscreen turned black after a short moment (1sec) and became unusable.
Many reboots later trying to be quick enough to enter device password before screen going black, i realized, that display brightness setting was the problem.
Solution: use the short moment you have to pull down the notification drawer and tap the brightness toggle somewhere on the right.
Strange problem, no idea what caused it, but it took me quiet some time to fix, because I had to reboot over and over and didn't realize it was display brightness so I wanted to share.