You may be experiencing any of these issues:
The SIM card may not be recognized by the phone, or may have been positioned incorrectly in the phone. Follow these steps to validate the SIM card:
Your phone may have lost its connection with the network. Follow these steps to refresh the connection:
Some issues can be resolved by power cycling your phone. Follow these steps:
Safe Mode prevents third-party applications from functioning and is useful when troubleshooting issues caused by installed apps or services. See How to boot into Safe Mode to check if signal quality improves.