Customers trying to log on and check their account details were instead met with a downtime page which said that the bank was updating that section of the site.
A spokesman said: "We have withdrawn this element of our service whilst maintenance and testing work is being completed. We expect this part of our service to return to normal in the near future."
The service has now returned to normal.
During the period of downtime Capital One's product information and account application services was still available on the site and customers could access their account details over the telephone.
But one disgruntled Capital One customer who contacted ZDNet UK sister site silicon.com after being unable to access his account described the unavailability as a "disaster".
"I will be cancelling my card, that's for sure. If they can't keep their Web site up, I will lose all faith in their security and integrity," he said.