If you can't access your messages via one way, i.e. via the app, try setting Messages up as a widget and accessing it that way, or vice versa.

There's a size limit on the database for SMS, so, if possible, make sure you're within that amount. It runs into 1000s, but if you're well within that, it shouldn't be a problem.

I'm assuming your problem is related to the SMS database on your phone, not on the SIM card (Messages > Menu > Settings > Text Messages > Manage SIM card messages).

As your issue might be related to the SMS database, e.g. it's become corrupted, you may need to erase it and start again.

First, if you want to keep them, back up your messages. See Android Market to install one that fits your needs.

Then Menu > Settings > Applications > Manage applications > All > Messages > Clear data. (If applicable, do this for Messages Widget, too.)

Do the same again, as above, for MessageCS.

Restart your phone.

If this doesn't clear the problem, you may have to consider doing a factory reset, in which case you will lose all your personal data.