What exactly is happening? What does the error say?
Do you get a loading screen? Do you see a black page and gray frame?
Do you see a white page with nothing on it?
Does your game have sound? If so, can you hear it?
What does it do on other browsers?
What version of browser are you running?
Are your video card drivers up to date?
Do you even have Java installed? If so what version?
What OS are you on?
COME ON MAN, give us some info!!