Restore a backup of your game from before you made that change.
I believe your buildings were destroyed due to decay being enabled on Decadent (though it’s not supposed to be). There’s a bug where decay kicks in instantly and destroys everything if you enable it some time after starting a game. I think this was recently fixed in the Testlive build.
On SP the save files are located here:
\steamapps\common\Conan Exiles\ConanSandbox\Saved
It should be the same or similar for dedicated servers, I would imagine. Depends on how you set up your server.
Delete or rename game.db to something like game.db.bak and then rename one of your backup files to game.db. Check the timestamps of the backups to determine which one to rename and load.