Lumia screen turning off during call

For some reason when I'm in a call the screen is completely black and I can not get it back on despite taking it away from my face and pressing the power/lock button. Is there a setting I'm missing here? What's going on here? It just started doing this.

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This could be one of two things:

- did you recently install a screen protector that covers the proximity sensor? If so you'll need to either cut a notch out of it so it doesn't cover it or take it off.
- There is dirt on the phone screen and it's on the proximity sensor. Clean the screen!