Try the following in order: (Note: If any step resolves the issue, STOP, do not go on to the next step.)
- Try removing, and then, re-installing the SIM card again.
- 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.)
- Reset your iOS device's Wi-Fi settings: Settings > General > Reset > Reset Network Settings
- Perform a "Reset All Settings" reset: Settings > General > Reset > Reset All Settings
- Contact your cellular service provider for a new card.