Details - Hotmail/Outlook.com outage on March 12th
At 13:35 PM PDT on March 12th, 2013 there was a service interruption that affected some people's access to a small part of the SkyDrive service, but primarily Hotmail.com and Outlook.com. Availability was restored over the course of the afternoon and evening, and fully restored by 5:43 AM PDT on March 13th, 2013.
On the afternoon of the 12th, in one physical region of one of our datacenters, we performed our regular process of updating the firmware on a core part of our physical plant. This is an update that had been done successfully previously, but failed in this specific instance in an unexpected way. This failure resulted in a rapid and substantial temperature spike in the datacenter. This spike was significant enough before it was mitigated that it caused our safeguards to come in to place for a large number of servers in this part of the datacenter.
Full post at the Office Blog