Do you block the person you blocked? Try the easy option by go to Messages > Click 3-dotted icon near the search button > Settings > Blocked numbers and Messages > Block numbers and see how the blocked numbers you had block.
Do you have Samsung Cloud? Samsung Cloud is having on Samsung phones according to your device. You get 15GB of Storage and You can purchase more. You can back-up data, Syncing photos into Samsung Cloud, and store files into samsung cloud. Even if your phone has been lost, replaced, damaged.
* Samsung Account required
Go to Settings > Accounts & Backup > Back-up & Restore > Restore data
Deselect all and select messages. Depending what you back-up each content. Wait until the restore is complete. Once finished restoring, You can view retrieved old messages in the Messages app. If not, Please look the apps that can retrieve old messages at Google Play Store.