Well it might be your xbox, internet connection, or disc because nothing is wrong with Modern Warfare 3. Everyone is playing. You might try and visit the xbox website and go to the support section because there are plenty of helpful topics there.
Also, try other games, if its just MW3 then you know it's the disc. Try downloading it to your hard drive if you can. If that does not work you might just have to buy another disc could because it's corrupt.
I Hope this helps. Good luck with fixing it.