Currently Being ModeratedJul 19, 2013 1:17 PM (in response to glyniphone4)
1. Try a Restart by pressing the sleep/lock button until you see the slider. Slide to power off. Restart by pressing the sleep/lock button until you see the Apple logo.
2. Try a Reset by pressing the home and sleep buttons until you see the Apple logo, ignoring the slider if, it comes up. Takes about 5-15 secs of button holding and you won't lose any data or settings.
Currently Being ModeratedJul 19, 2013 1:31 PM (in response to glyniphone4)
Usually indicates the proximity sensor is not functioning correctly
If paulcb advice does not solve it try a full restore first with a backup and then if required as a new device
If it still it is an issue it is a hardware problem and you will need to take it to an Apple retail store Genius bar