Close All Open Apps... Perform a Reset... Try again...
Reset ( No Data will be Lost )
Press and hold the Sleep/Wake button and the Home button at the same time for at least ten seconds, until the Apple logo appears. Release the Buttons.
The Basic Troubleshooting Steps are:
Restart... Reset... Restore from Backup... Restore as New...
Restart / Reset
Backing up, Updating and Restoring
If you try all these steps and you still have issues... Then a Visit to an Apple Store or AASP (Authorized Apple Service Provider) is the Next Step...