1. Do a hard reset (Hold down the power and home button simultaneously for about 20 seconds till you see the Apple logo)
2. Turn off the phone for about a day and turn it back on to see if it works.
3. If nothing works, get a warranty replacement (if you still have warranty left).
4. If you don't have warranty, the only option is to go for a repair. Visit an Apple store, they'll tell you what to do next.