Restoring Missing iPhone Messages
You should try accessing the SMS db from the command line. Some explanation is here: How can I export SMS text messages from my iPhone?

If you see old messages, you can find the corrupt one and retrieve them. If the Sqlite dump command doesn't show them, and the size of the DB implies they are not there, it is likely they are gone.

If you can find old copies of that file in backups, look there, otherwise I think the messages are permanently gone.