Does it have a SIM Card? If it does while the phone is on take out the Battery & SIM Card for a minute or so then replace and try it.
If it doesn't then you have to call customer service & have them clear by our switch
You could also try a PRL update, if it is a CDMA phone, sometimes a quick update will help jog the network and help it find its way.
If it is a SIM phone, like everyone has been saying, turn it off, battery out, and back on again. Try sending a message to yourself. I've had nervous break downs because I thought my phone wasn't getting messages when, just simply no one was texting me back. ;)
dial *22803 + SEND. Press #1 for English and #2 to update the PRL (preferred roaming list).