Sounds like a disk reading error to me. Clean the disk off, make sure it is free of dust, dirt or anything else that would stop the laser from reading it correctly. If after you have cleaned the disk it is still not reading correctly check the disk for scratches, even a small scratch can cause a reading error. If the disk is scratched, you could attempt to repair the disk using something like a disk-doctor. If the disk still does not work correctly, than you may have to clean your Xbox's laser, console's lasers do get dusty every once in a while, and cleaning them may clear up the error. If none of this works, then it's time to get a new disk. If you bought the game recently, return it for a new one. If not, unfortunately you will have to buy a new one.