Have you tried reseating the SIM card? If that doesn't help, you may need a new one; they're free at your local Corporate VzW store or by contacting Customer Service.
Can you tell me how to reset the SIM card? Will it erase all my data from my tablet itself? Thanks!
Reseating a SIM card won't remove any data/info. Turn off the tablet, remove the SIM card, wait a minute or two, then reinsert the SIM card, power the tablet back on, and allow it a minute or so to establish a fresh network connection. In case you need it: