Q: my iphone?

Have you tried removing it and reinstalling it?

Have you tried a restart?

Reset?

Restore?

Try to take out and put your SIM card back in if you have not already (this is referred to a reseating the SIM).  If that does;t work, try getting a new SIM form your carrier.  They can either get you one in a store or mail you one if you call them.  Here are some other steps you can try as well: If you get an alert about the SIM card in your iPhone - Apple Support