in's Creed II gets stuck. help!?

I'm in the second mission. At the part where you have to go to Leonardo's house and he fixes the hidden blade. The screen just freezes. I can still access the menu, but the main game doesn't do anything. All I can see is his shoulder. Does anybody know what I can do? Or is this a faulty disc and I need to get it replaced. If yes, what do I do?