All the code, functions, variables, ect, you’ve made in your GameStateBase need to be recreated in your new GameState. Because you’ve set the new GameState into the GameState field for the World Settings’ GameMode, that World will now only use your new GameState. All the references and casting you’ve called in your other blueprints to GameStateBase need to be redirected to the new GameState you created (that’s probably why your game ceased to function; the old GameStateBase is not valid because the new GameState is what is loaded via the World GameMode settings.)
Every World you travel to will need the new GameState set into the GameMode settings.
Essentially you need to swap the old GameState with the new one everywhere it is referenced.
I recommend making a backup of your project before troubleshooting. It will save a lot of grief if after all the adjustments things are more broken than before and you’ve forgotten all that you’ve changed.