First off, if you haven't checked the gravity then you can't be 100% sure that it's not fermenting. Sometimes you won't be able to see visible signs of activity and even just a tiny air leak will prevent your airlock from bubbling.
That being said, if it really hasn't started fermenting at all then the first thing I would check is to make sure that the grape juice you used didn't have any preservatives. I think I remember seeing that sorbate and benzoate are two to look out for.
If that's what happened, if you go down to post 13 on
this thread it will give you some good instructions on what you can do.
Otherwise, some possible problems are your temperature being outside the yeast's range or pitching when the must was too hot (if you heated it at all).