Sooooo annoying! This has been going on, off and on for the past month. I will randomly be having a conversation through text message with someone, and then it will suddenly stop. I have 4 bars, I can make phone calls, go on the internet and do everything else just fine. But i can't send texts messages, even though I can get them from people. And then, it will just start working again. Then a little bit later it will act up again. I have tried restarting my device, shutting it off and taking out the sim card. Nothing fixes it. It only starts working when it decides it wants to haha. Its getting very frustrating though!
How many text messages do you have saved? There is a limit to the number of texts and mms (fewer) per conversation and this can be changed in settings. Unless the problem lies with Verizon, get rid of some text messages, clear the messaging cache, and reboot the phone. A system cache clear is next. Then a Factory reset as a final resort.
One additional thought if you are still having texting problems, after clearing out your old messages go to Settings > Application Manager > All > Messaging > Clear Data > OK and then reboot your phone.