As we known, every time you sync iDevice with iTunes, it will automatically make a backup of your iDevice contents including photos, SMS, reminders, contacts and more. Therefore, as long as you have ever synced your iPhone with iTunes, you can get the lost text messages from iPhone backup files created by iTunes.
Do not connect your iPhone to iTunes again until the messages are recovered. Otherwise, the old backup file will be overwritten by the current content on your iPhone. Once it happens, the deleted messages will be gone forever.