Texts don't just vanish. They are either deleted, the cache for the text messaging app is cleared or the phone is reset. If it is the last one, they are gone. If it is from a different app (not the phone's text messages) then you would have to contact the apps creators to see if it is possible to restore the messages. If it was the phone's text messaging app, then you 'may', and I do only mean 'may', be able to get them back.
While there is no guaranty that you can get them back. Deleted does mean deleted. There are a few apps/programs out there that 'may' be able to help you recover the data.
A few of the programs you could try would be... DiskDigger, MobiSaver, Android-Recovery and Tenorshare. They are all data recovery...