Nationwide Building Society's online banking hit
Customers of Nationwide were unable to access account information after computer problems struck at the building society on Tuesday.
Online banking was unavailable and some customers were unable to access account details in branches.
The building society apologised and said it was monitoring its service closely.
It is not the first electronic failure to hit Nationwide. An error led to double payments being taken last July.
Some 704,426 account holders saw money taken from their accounts twice on that occasion. Nationwide said that was caused by a "one-off human error".
Nationwide, the biggest building society in the UK, has about four million customers.
Customers complained that some services were down for much of the day.
The mutual, owned by its members, said that an IT issue affected services including its online bank. A message on its website told customers that online banking was unavailable.
The problem was solved by late afternoon.
"We are pleased to confirm that the problems which have affected our online bank have now been rectified but we will continue to closely monitor all services," said Jenny Groves, divisional director at Nationwide.
"We sincerely apologise to any customers who have been inconvenienced. We very much regret falling short of our normal level of service."
The problem comes a week after customers of Lloyds TSB and Halifax reported problems accessing their accounts via the internet.