If you want your game original ....

Remove any mods, uninstall any mod managers you may have installed. Those can mess up your game again.

Uninstall game.
Manually go to the install folder and delete the entire folder.
<your saves are in appdata so they should not get messed with but it never hurts to have a backup.>
Reinstall game

You must manually delete the folder to remove mod files and any corrupted files. An uninstall will not remove files it did not put in.

Start game it should be back to vanilla.


You can't solo a server world on your machine without some outside work.
If you don't have access to the server the world is probably gone with it. World saves are server side.
If you just have non server saves they should work normally now but any modded content will be missing. Not sure how that may effect your save files.

Friendly reminder that modding is done at your own risk.
Good luck!