Sounds like you might want to check the console (F12) while the game is running. If Superpowers runs into an error (like changing the health of an actor that doesn't exist), or an infinite loop (for, while, etc...), it will sometimes freeze when the loading bar is full, but before the game starts. Usually it'll show an error, a script name, and a line number that you can use to find out whats going on.
Hope this helps!