You may have done this by now, but I would suggest restarting your console. If that doesnt work try a different multiplayer game, then go back to COD4 and see if you can join a game then. And finally restart your modem and/or router.
To do this
Make sure your console is off
Unplug your router followed by your modem
Wait for 30 seconds
Plug in your modem and wait for it to fully light up (15-30 seconds)
Next plug in your router and wait for it to fuly light up as well
And lastley turn on your console
Also, test your xbox live connection and make sure your nat is open or moderate.
Hope this info helps.
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