The Basic Troubleshooting Steps are:
Restart... Reset... Restore from Backup... Restore as New...
Restart / Reset
http://support.apple.com/kb/ht1430
Try this First... You will Not Lose Any Data...
Press and Hold the Sleep/Wake Button and the Home Button at the Same Time...
Wait for the Apple logo to Appear...
Usually takes about 15 - 20 Seconds... ( But can take Longer...)
Release the Buttons...
If that does not help... See Here:
Backing up, Updating and Restoring
http://support.apple.com/kb/HT1414
If you try all these Steps and you still have issues... Then a Visit to an Apple Store or AASP is the Next Step... Be sure to make an appointment first...