Having a pretty important issue with screen turning off right after turning on.

I've found that if I'm in a dark room and want to turn my screen on, it turns off right away after hitting the power button. I know about the proximity sensor and I'm not covering it with my hands, so I'm wondering if it's my screen protector/phone case that is making it not able to turn on for longer than one second in any dark room. I really hope this issue isn't unfixable.