First, try a system reset. It cures many ills and it's quick, easy and harmless...
Hold down the on/off switch and the Home button simultaneously until the screen blacks out or you see the Apple logo. Ignore the "Slide to power off" text if it appears. You will not lose any apps, data, music, movies, settings, etc.
If the Reset doesn't work, try a Restore. Note that it's nowhere near as quick as a Reset. Connect via cable to the computer that you use for sync. From iTunes, select the iPad/iPod and then select the Summary tab. Follow directions for Restore and be sure to say "yes" to the backup. You will be warned that all data (apps, music, movies, etc.) will be erased but, as the Restore finishes, you will be asked if you wish the contents of the backup to be copied to the iPad/iPod. Again, say "yes."
Finally, if the Restore doesn't work, let the battery drain completely. Then recharge for at least an hour and Restore again.