I've heard this happen to a few people. I'm assuming it could either be because of you upgrading your OS, hence getting a new version of the messages app. As always, do make sure no one (kids?) got into your phone and deleted them. Hopefully you had them synced. Try restarting your phone. If that doesn't help, go into settings > clouds and accounts > Samsung cloud > check and see if messages data/all phone data is backed up/synced. If not, try clearing messages app cache by going into settings > apps > messages > storage > clear cache. You can also try contacting your service provider, but unfortunately, I don't think there's any way to get them back. Do make a note of feedback to Samsung, though. Hope that helps.
Have a look at this link - FYI I use SMS Backup & Restore from the Play Store - It backs up my SMS to my Google Drive account on a daily schedule - I don't know if the link below will work for you - But it's a good idea to have your important information backed up daily 🐧
