Customers took to Twitter to vent their anger after many were unable to withdraw cash or transfer money online.
NatWest customer Mark Hillman had planned an evening out last night, but it was cut short when he was unable to withdraw any cash.
He said: "I went from a night out to staying in, because I couldn't get my hands on my cash."
More top news
A hacker group who claimed to be behind a cyber attack on Sony Pictures have posted a message online apparently mocking the FBI.
The returns of Benedict Cumberbatch's detective and the X Factor judge were among 2014's most tweeted TV moments in a billion conversations.
A family has issued an appeal to help find their missing pet turkey - in case he gets gobbled up as someone's Christmas dinner.