Try rebooting your phone and wait a minute or two before opening the messages app then click a few times on the center of the app and your messages should start appearing...

Anyway the most important thing is making sure not to lose any messages so run N9QT Tweak NN-2 and export all of them first...

As long as they are in the database they will get exported even if they don't appear in the messaging-ui...

You can also try exporting using Nokia Suite but the above method is what you should try first...