What those guys said...the proximity sensor thinks it is still up against your ear so it turns the screen off so you don't accident poke buttons whilst yer chatting. Mine has recently started doing this. I've found if I squeezed my phone, front to back, right under the front camera, thumb on front, forefinger on back, the screen would come back on...I downloaded the Phone Tester app and the proximity sensor was always 'near' until I did the squeeze then would change to 'far'. When you have trouble waking up the phone, even using the button, this works too. I got my phone the first week it was available and has only recently started doing this. The squeeze 'fix' makes me think the sensor is not quite close enough to the screen for whatever reason.