Albright said the site undergoes "regular nightly maintenance" during off-peak hours and that period was extended because of a "technical problem." He did not say what the problem was, but a statement from the Department of Health and Human Services called it "a software bug" unrelated to application volume.
Albright said consumers who left an email address would be "invited back" when the system got up and running again.
Officials said the website wasn't hacked. The site, which was receiving 1.5 million visitors a day last week, received about 1.7 million on Sunday. The federal site serves 36 states. Fourteen states and the District of Columbia are running their own sites, some of which have been crippled by technical problems.
Nonetheless, the administration and the states appear to be on track to sign-up about 6.5 million people for subsidized private health insurance through the new online markets. That's half way between a revised goal of 6 million and the original target of 7 million. The earlier goal was scaled back after the website's disastrous launch last fall, which kept it off-line during most of October.
It is unclear how many consumers who have signed up ultimately closed the deal by paying their first month's premium. Also unknown is how many were previously uninsured — the real test of President Barack Obama's health care overhaul law.
Albright said Monday morning that the website is typically down for maintenance during the period from 1 a.m. to 5 a.m. EDT, and that as a result of the technical problems the site was down for close to four additional hours before returning to full strength.
The sign-up website had been taken down briefly Friday, with consumer interest surging.
A recent analysis for The Associated Press by the performance-measurement firm Compuware found that the government site runs slow compared with health insurance industry peers.
With long lines reported at centers offering in-person assistance to enroll, HHS Secretary Kathleen Sebelius was leading the deadline day sign-up effort. Sebelius planned to spend much of the day Monday working out of the department's TV studio, conducting interviews by satellite with local media stations around the country.
Although March 31 was the last day officially to sign up, millions of people were potentially eligible for extensions granted by the administration for various reasons.
That includes consumers who had begun enrolling by the deadline but didn't finish, perhaps because of errors, missing information or website glitches. The government says it will accept paper applications until April 7 and take as much time as necessary to handle unfinished cases on HealthCare.gov. Rules may vary in states running their own insurance marketplaces.
The administration is also offering special extensions to make up for all sorts of problems that might have kept people from getting enrolled on time: Natural disasters. Domestic abuse. Website malfunctions. Errors by insurance companies. Mistakes by application counselors.
To seek a special enrollment period, contact the federal call center, at 1-855-889-4325, or the state marketplace and explain what happened. It's on the honor system. If the extension is approved, that brings another 60 days to enroll.
Associated Press writer Connie Cass contributed to this report.