Halifax's automatic teller machines on its 800-branch network were down for much of Monday afternoon last week, according to a bank spokesman.
The Halifax said the storage disc in the server that connects the bank's ATM network with customer accounts systems failed, causing the ATM network to become overloaded.
Analysts have expressed surprise that the Halifax did not have contingency arrangements to prevent such a failure bringing down its ATM network.
"ATMs are normally fairly reliable, as it is a closed network system and relatively little work is done with them," said Daniel Mayo, lead analyst in the financial services practice at analyst firm Datamonitor. "With a storage disc you would normally expect to mirror a system with two sets of data, should you have hardware problems."
A spokesman for Halifax said the bank had tried and tested contingency plans, which included using a number of datacentres.