Are you specifically referring to SMS/MMS or iMessage? Here is a support document about troubleshooting Messages. http://support.apple.com/kb/TS3367
Also, if none of this helps, you may want to contact the carrier, as SMS/MMS is a carrier function. If it is iMessage you are referring to, then check to see it is activated.
Sorry, that was an incorrect link. This is the correct one. http://support.apple.com/kb/TS2755
Message was edited by: ChrisJ4203