The site was down for maintenance Monday morning.
Healthcare.gov was down for maintenance Monday morning, and is now queueing guests — on the last day to attempt to enroll for health insurance, though users who say they tried to enroll (but were unable to) can apply for a special enrollment period. As Politico reported this morning, the site was down from 3 a.m. EST until sometime after 7a.m. The site undergoes nightly repairs, and the Department of Health and Human Services officials said that a fix on a software bug extending in to high traffic hours. Around 7:47 am on Monday the "system is not available" message switched over to the queueing system. Part of the reasoning behind the White House's recent (and controversial) special enrollment period extension was that it expected the site would get overloaded, as it is right now.
Healthcare.gov has a long history of being down on really important days. Most recently it was down on February 12, aka National Youth Enrollment Day. It was also down for most of October. (The White House's enrollment countdown is also a little glitchy — it's counting hours down in the days column.)