For much of the day, customers at Britain's largest building society were unable to withdraw money from cash dispensers or get access to the society's web site to pay bills.
The problem started at around 9.20am. Engineers initially thought that it was a major software failure only to find some hours later that the cause of the chaos was a fuse box at the Nationwide's Swindon headquarters.
Meanwhile, customers who needed to lay their hands on cash had to process their transaction over the counter.
A spokeswoman for the society admitted: "A minor bit of hardware wore out. We've had to replace a couple of fuses. We are extremely apologetic." She insisted that the breakdown had nothing to do with Y2K, the millennium bug problem.
John Turner, a computer consultant who was one customer unable to access his account said: "In the event it was a minor problem with no serious consequences. Had this been the fourth of January, we could have had a major panic on our hands."