I had this problem too, search for my thread that might help.
Anyway, what I did that helped, is that I took it out and just put it back in. There might be dust blocking it, or it might just need to re-recognize it, or both! Trust me, I basically started sh!tting bricks when that happened. Just fool around with it. In worst case scenario, you might need to restore it. But make sure you save all you pics and goodies before you do that.