Basically everyone else is right. Take the battery out, cuz it's like going through a shock mode(I dunnoh XD) But, keep it out for a few moments then, put the battery back in. If this doesn't help..either get it replaced, if you have warranty ;) or go to your dealer and see what they can do. It could cost a lot if bad condition.
The same thing happened to me. Really the only thing you can do is turn it on and off and if that doesn't work, you'll need to go to a phone store or a place that can repair it.
Normally when that happens I just take the battery out and turn it back on. If it doesn't work, get it repaired.