Have you tried reseating the SIM card? If that doesn't work, the SIM card may have become corrupted; you can get a new one free at your local Corporate VzW store or by contacting Customer Service.
That was an issue with the M7 model of the HTC One line. It hasn't been widespread issue with the later models. The OP didn't mention which phone model they have, only the manufacturer name.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
Did you try restarting and resetting the phone? It might be something that went wrong while the phone started up. Water damage in the SIM card socket may also cause this problem.