What I think that happened is that by placing it in your addons folder and starting the game, it installed the UI in your Blockland folder.
So even after removing the addon, it is still installed there.
What I would do is install a fresh copy somewhere else and copy the base folder to your real Blockland folder.
Maybe some more folders need to be copied, but I am not sure.
Then the UI should be restored.
If that doesn't help, better reinstall the game.