People attempting to use Halifax and Bank of Scotland cards faced frustration after a power failure affected the bank's systems.
All the bank's cashpoints went down and there were also problems with authorising transactions in some shops.
HBOS, which has 10 million customers, said it was working to restore services.
A spokeswoman said: "Due to external power disruption in Yorkshire, which has impacted one of our IT centres there, some Halifax and Bank of Scotland systems have been affected this morning.
"Power has now been restored to the centre via our back-up generators.
"We are currently in the process of getting services back to normal.
"Our branches are open to support customers as usual. We apologise to customers for the disruption and inconvenience this has caused."
The "vast majority" of transactions in shops should work as normal, she added.
