It looks like Microsoft is having some issues with its Xbox Live service this evening. People have reported they are unable to sign onto the online gaming service and Microsoft's Xbox Live support webpage confirms the issues.
The status page claims the problems are affecting Xbox 360 owners, along with Windows 8, Windows Phone, Games for Windows Live and even the Zune PC client. Microsoft's message states:
It looks like you’ve been disconnected from Xbox LIVE and worse, you can’t sign back in! We will get this fixed as quickly as possible. Thanks for being patient while we sort this out. Please check back in half an hour for more information.
Hopefully this will be solved quickly. We will naturally update this post when more information comes in. The downtime did come at the same time that Activision released the Revolution DLC pack for Call of Duty: Black Ops II for download.
Update (9:15 p.m. CST): Xbox Live appears to be back up for some people... although the status page is still showing downtime, so your mileage may vary
Update: (9:55 pm CST): The status page now says Xbox Live should be back up for everyone now, so you can enjoy playing your Revolution DLC.
Source: Xbox Live status page