Check your network connection.
Try these steps:
- Restart your device.
- Sign out and back in to your account.
- Try to reset your device. This won't delete your data stored on it.
- Press and hold the Sleep/Wake button
- Press and hold on the Home button
- Keep holding both buttons until the display turns off and back on with Apple logo on it.
Alternatively, you may go to Settings - General - Reset - Reset All Settings
https://support.apple.com/en-us/HT201559