Microsoft has been having problems with both Outlook.com and SkyDrive. Both cloud services have been out for some users for over three days, though Microsoft are now saying the online services are back up and running.
According to Engadget Microsoft say the downtime was due to Microsoft Exchange issues:
Trouble began with the failure of a caching service for Exchange ActiveSync. The resulting deluge of reconnection attempts promptly overwhelmed company servers; a slow recovery was necessary to avoid another meltdown