Have you tried resetting the SIM card by turning off the phone, removing the SIM card (be careful not to touch the gold contacts), waiting a few minutes, then reinserting the SIM card, turning the phone back on, and allowing a few minutes for a fresh network connection?
If that doesn't help, SIM cards do fail from time to time. You can get a new one free at your local Corporate Verizon store or by calling Customer Service.
