Quit the App by opening multi-tasking bar, holding down the icon for the App for about 3-5 seconds, and then tap the red circle with the white minus sign.
Relaunch the App and try again.
Restart the device. http://support.apple.com/kb/ht1430
Reset the device. (Same article as above.)
Restore from backup. http://support.apple.com/kb/ht1766 (If you don't have a backup, make one now, then skip to the next step.)
Restore as new device. http://support.apple.com/kb/HT4137 For this step, do not re-download ANYTHING, and do not sign into your Apple ID.
Test the issue after each step. If the last one does not resolve the issue, it is likely a hardware problem.