Looks like Microsoft is the latest company experiencing an outage, as they are working through an issue with Xbox Accounts and Xbox Live per their Xbox Support Twitter. Apparently, some users are having issues accessing their Xbox Live Account along with owned games and Apps:
The issue appears to have been going on for a little over 30 minutes so far, so hopefully this is something that can be addressed quickly. It also looks to be impacting Xbox on Windows titles as well, so keep an eye on the official Xbox Live Status page
or the Xbox Support Twitter page for updates.
[UPDATE: DAN] Looks like services are back up and running. You would have thought that Microsoft was tapped into the AMC Theater and Endgame ticket frenzy today, as the serveries were down almost for teh same time frame...