You don't mention what device you have, so the advice below might be not quite correct for you. If you can't figure it out, repeat your question in the forum for your device.
Anyway, go to Settings / Application manager. Find the list for "All apps." Scroll to the messaging app. Tap there, and tap on Clear cache. Then tap Force stop, and exit settings. Restart the messaging app, and see if it's working correctly now.