We were down for about 4 hours on Monday night due to an outage at our ISP.
While our uptime is generally very good (~99.8%), I apologize for the inconvenience here.
UPDATE 4/11/2013: It turns out the problem was a loss of electrical power, caused by an electrical fault during maintenance.
As the faulty electronics caused smoke to be emitted, they had to call the fire department and wait for the room to be declared safe before they could turn everything back on.
That's why everything (including the ISP's own website) were down for a few hours.