Try the following in order: (Note: If any step resolves the issue, STOP, do not go on to the next step.)
- Power the iOS device OFF, then ON.
- Restart your iOS device: To restart your (perform a soft reset) iOS device, press and hold the sleep/wake button while pressing and holding the Home button. When you see the Apple logo, release both buttons. (Note: This will NOT delete any data or apps, nor change any of its settings.
- Remove, and then, re-install the SIM card.
- Restore your iOS device: To restore your iOS device, connect it to your computer. When the iOS device appears in the iTunes source list, click the Restore button on the Summary tab. (Note: The following will erase all of your data and media and resets all of the iOS device's settings. You will need to restore it from a previous backup from either iTunes or iCloud.)
- Make a Genius Bar appointment at your local Apple Store or take it to an Apple Authorized Service Provider.