Try this:
Make sure IOS is updated to latest version
(How to update your iOS device
http://support.apple.com/kb/HT4623)
Reboot device by pressing both the home button and sleep/wake (power) buttons at the same time for 10-15 seconds until the apple logo appears on the screen, then let go.
If that doesn't work then reset the device by going to settings/general/reset/reset all settings (or for network connection issues 'reset network settings').
(no media or data will be deleted from the device, this will only take a minute).
If that doesn't work then backup and then restore the device as new first (to isolate cause of the issue). Then from backup if necessary.