Might not be the problem but..
I had my Wii Sports disk facing bottom-up on top of my entertainment center. I left the house for a few hours, came back, put it in and it didn't work. I looked at the CD and saw a bunch of little spots on it, seems that my mom sprayed Febreeze, which doesn't scratch the disk but does make it unreadable..
Just try wiping it off gently, unless you were just putting it in the wrong way. ;p