I have no idea what would've caused it, and I doubt there is anyway of getting them back (unless you made a semi-recent nandroid backup). This is actually the first time I have heard of this on these forums. Unfortunately, with text messages, Android is designed to allow other applications to access them (so you can use 3rd-party messaging apps to recover your android phone number). This could mean it could be an app that accidentally deleted them, not necessarily Android.