Why is Black Ops Multiplayer not working on xbox?

Xbox Live and Zombies is not working, I push the ready up on and then people join and I can hear them, but then when the countdown is over it says Lobby closed, and I back where i started, WTF??!! I'm a noob at COD so..... My gamertag is ExuberantMonkey if you wanna play a couple rounds Halo Reach or Black ops. Thanks