This happened to me, the game was installed on my hdd, but still didnt work. I just rebought my disk.

If your disk is scratched, the only of fixing it is using a Disk Cleaner or going to your local Gamestore to get it fixed.

If the disk is dirty, try using the toothpaste trick. It usually works for me!

Or...if your Xbox won't read disk's, try putting the Xbox on it's side or get the disc tray fixed as said above.