Make sure IOS is updated to latest version
(How to update your iOS device
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.How to back up and restore (iCloud and iTunes)