Gary was right on, unplug the battery for a minute then try a RESTORE directly through iTunes and make sure to update to the latest iOS (don't forget to back up and make sure you are aware of the iCLOUD ACCOUNT AND PASSWORD attached the phone before doing this). I've seen this issue a lot. Most of the time it's from liquid damage or logic board damage and not from a faulty camera, but your camera could also be defective. I guess the LED could be shorting out if it was touching the logic board. If the screen replacement was done recently I would bring it back and ask them to test another camera in the phone just to rule out of the possibility of a faulty camera.