Dismantle the phone to check the damage. if nothing inside then just restore it with iTunes. it be a software issue. put the phone into the recovery mode by steps:
turn off the phone.
Hold the home button then put the cable inside he phone and pc.
it will show the iTunes logo and cable on the phone.
Restore it pressing restore button.
It need a latest file to restore . latest firmware is 5.1 . if you do not have just press the restore button it will automatically download from the apple site.
other way is to put the phone into DFU mode to restore it.
put the cable inside the phone and pc.
PRess the Home button and power button together after apple logo on the screen
after 3-4 seconds just release the power button but keep holding the home button.
after approx 8-9 seconds you will see iTunes detects the phone.
Then you are ready to restore it with same button .. best of luck