For the second day in a row, Xbox Live is experiencing issues and is down for some players across the world. At the moment, the outage isn’t universal, but it’s patchy and impacting players in different regions, from South America to parts of North America. The main issue seems to involve players unable to sign-in to their accounts, and thus play their games linked with said accounts. In other words, some players can’t play any of their games. Microsoft is aware of the issue, and is working on a solution as we speak.
Source: N4G PC Xbox Live Down and Experiencing Issues For Second Day In A Row