I’ve just had a really infuriating half an hour trying to figure out why I couldn’t get the keypad to appear during a call on my Lumia 920. When I took my phone away form my ear the screen stayed black. Pushing the power button made the display switch on and then immediately switch off. Power cycling and even resetting made no difference.
A close examination showed that the small round circle next to the speaker slot at the top of the phone display was full of dust. Exactly how this happened I am not sure – you’d expect that bit to be sealed, wouldn’t you.
Five minutes with a compress air cannisted blowing into the speaker slot and headphone jack socket later I had managed to blow the gunk out of the proximity sensor an as if by magic, the keypad works like it should again.