Xbox 360 call of duty 4 problem?

please help! i have a problem with call of duty 4. i can start it up, play it in singleplayer, but as soon as i try to go in to multiplayer it brings this message: the disc is unreadable. i have tried calling the support center, and they told me to take off the harddisc, and i did. it work ones, but now it doesn't. do any of you have the same problem, or does any one know what i have to do?