IPhone 4S suddenly isn't sending texts overrm cell, what to do?

It was working fine, until a few hours ago

I've got a Windows phone, and it's done that a couple times. Have you tried turning it completely off and then turning it back on? That worked for me.

Is your data communication still up? If so, just try an app like PINGER to send free text messages. You might want to check with your service provider as well. You might, or they might, have a problem.