Ok, same thing happened to my husband and one of our friends, we called microsoft xbox support, they told us we'd have to call the games company to get any help because they just deal with the xbox system and components.... so we called activision/infinity ward 310-255-2050 and we used the www.activision.com/support
site a few weeks later and this is the answer we got: they have no fix for the problem (even though it's been a problem for several months now) and although you can no longer just find a game on multiplayer or even start a private match, you can still be invited to play by someone else, and then you can gain xp normally
(unless god forbid you end up in another hacked server, sigh) and you can work off the negative xp... eventually you're supposed to be able to play normally again after that, we haven't tested it out, since the other friend we had that plays is also negative on xp. But we also aren't buying anymore of their games until they can fix the old ones first. hope this gives you at least a glimmer of hope, or at least the number and website to their customer service.