Check whether the phone was synchronizing with gmail/google. Search the associated Google account for any known txt message content. there may be an associated tag/metadata property identifying txt messages so you can sort them out from email.
Most SMS backup utilities in Android either back up to a file on SD card or to your gmail account as individual items.
Failing the above working, connect your Galaxy phone to your laptop and use an Samsung Data Recovery
program to be able to view the deleted files in the task window. After this, click on the messages that you want to recover.
How to recover deleted text messages from Samsung