It sounds like you have a damaged OLED screen. You'll have to replace the screen if you want to keep using the phone. Grab a flashlight and see if you can spot any cracks on the screen beneath the glass.
I have to agree with con. You're best bet would be to disassemble, clean, assemble, try a new screen, and see if the phone is recoverable, but it is possible the issue could be caused by damaged to the logic board.
It sounds like you're having an issue with your proximity sensor. If there is no damage to the screen or the proximity sensor, it is possible the issue is caused by your screen lifting. One way of checking is by applying pressure to the top of your screen while avoiding the 2 small circles where the proximity sensor is located during a phone call after giving your screen a quick cleaning.