what messaging app are you using? See if it needs an update.
if not you could try clearing the cache, though this of course erases your settings.
I have a "text message not sent" on the top message bar that countinuosly pops up. It first appeared when I was forwarding a picture through a text message. The error appeared on the message bar, and when I select it it goes to the list of recent text messages, but says "message not found". I since forwarded another picture, and now I have "3 text messages not sent". If I select this it goes to the text messsage list and the message clears. However, anytime I do anything associated with text messaging (i.e. send, recieve, view) the error re-appears.