please try the following:
1- Turn off the phone
2- Remove the battery from the phone
3- Remove the Sim Card (only if your phone has one)
4- Wait 5 minutes
5- Insert them all back and turn it on.
6- Does it work now?
7. if no try to hard reboot it.
Remove the battery (if you have on, varies by carrier) from the phone and wait 20 seconds, then, with the battery still out, plug it into your wall charger, then place the battery back in and turn the phone back on.