Yes it sounds confusing, but if I understood you right, you changed the default program for .exe files? Please correct me if understood wrong, but thats the only thing I can imagine you have changed. If I am wrong, please give me little bit more details about what is exactly happening when you will try to open the game.
To fix it you can open cmd with admin rights and type in:
in case that the command won't fix it, you can try to edit the registry as explained here: http://www.winhelponline.com/blog/exe-files-open-n...