The Xbox Live network seems to be totally down and out right now. Major Nelson re-assures gamers that the MS team are aware of the problem and are currently trying to resolve things. He'll post an update when he has one.
If you’re having some issues connecting to Xbox Live, or getting into a game (matchmaking) the operations team is aware of the issues and are working on it. I’ll post an update when I have one. In the meantime, hit the single player campaign of your choice while the team takes care of business.