If you've tried to login to Xbox Live recently to no avail… don't panic, it's not just you! Xbox Live is down for a lot of people, but Microsoft is aware of the issue and working on a fix as we speak.
Currently, according to the Xbox Support, the Xbox Live service itself is down, although you can still access your account and marketplace via the Xbox Live site or your Xbox Live mobile app.
Apparently, the Zune marketplace and TMZ app are down as well. I hope no one sheds too many tears there.
We'll update you as soon as there are any updates on the issue.