Have you recently installed or updated apps? One of those has likely caused this.
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Welcome to Android Central! Which phone? If it doesn't have a proximity sensor, or if you turned off the proximity sensor, this screen is in place to prevent inadvertent screen touches by your face while you're holding the phone up to your ear.
It's an lg10
It's not the proximity sensor(works normal), it's an overlay screen across the bottom third of the screen. It must be unlocked before hanging up the phone or to accesses the keypad.
No apps or updates lately.
I would like to post a pic of it so you could see it but I don't see an option for that.