Interesting thing... If you are using Windows, click "Win+D" twice. This will close and re-open all windows. You can also try to open it from the Downloads folder (or where you downloaded it to).

Hm, I can't start the game here on the site. I get this error: UnsupportedClassVersionError greenfoot/export/GreenfootScenarioViewer : Unsupported major.mino version 52.0 It works when I download it though.