Currently Being ModeratedNov 29, 2012 8:18 AM (in response to rbd3)
Is it always black, or only when you are on a call? If it is only on a call remove your case and screen protector (if any), as one of them is blocking the proximity sensor. If it is black all the time you probably have a hardware issue, but you can try rebooting - hold the HOME and SLEEP buttons at the same time until an Apple logo appears. If nothing at all happens after 30 seconds of holding your phone is broken.