1) Have you downloaded any apps that would have affected the text messanger app? These would be apps that require permission to send text messages. Look at the apps you've downloaded before you noticed this issue and uninstall each one to see if they are the culprit (install them back if they aren't).
2) Have you done a soft reset (restart phone) or a battery pull? You can try to see if it helps. As it's a stock app, you can't uinstall and reinstall it...unless you've rooted.
3) If #1 and #2 doesn't work, the only option I can think of is a doing a factory reset.
This is the only trouble shots I can think of. If there's something I'm missing, someone else, feel free to chip in.