This often happens with my 3Gs, I would suggest trying to message multiple people and try calling another phone. If the call will not proceed, then it is most likely you have been cut off or ran out of credit.
If however, the call is successful, try a hard reset (Hold the lock button and home button until Apple logo appears after turning off.) This is the usual fix with mine.
Other way I find help this, is to turn Airplane mode on, for about 30 seconds or more then proceed to turn airplane mode off. Wait until your signal bars return then try again.
Goodluck, and I hope this helped.