Xbox LIVE is currently down. According to the service status page, users may experience difficulties when signing into Xbox LIVE from the console or Xbox.com.
"We are aware of the problem and are working to resolve the issue," the message reads. "We apologize for any inconvenience this may cause and thank you for your patience."
The Xbox Support Twitter account adds: "We are aware of the issues some users are having with Xbox LIVE and are actively investigating. Thanks for your patience."
Microsoft hasn't said what the issue causing the problem with Xbox LIVE currently is, but it's good to know that they are working on it. We'll be sure to update you when we learn more.