A large number of banking and other leading sites have crashed, sparking a wave of angry customers unable to access money or use their services.
Barclays, Tesco, HSBC, and Lloyds are banks all affected by the technical issue.
Customers flocked online to complain, with many taking to Down Detector to vent their frustrations.
One said: "Both mine and my husbands cards were declined yesterday, despite us having money in our accounts. He put his card in one ATM, it spat it back out and said it had been detained. He tried it another and was able to withdraw."
Another said: "Been on 'hold' now for 25 minutes!"
A third added: "Unable to pay online, when called and tried making payment with the automated service it kept saying card number incorrect (I can see on screen its not and same I usually make payments with online).
"Then when 'I entered an incorrect debit card number' 3 times in a row the machine hangs up on me... Brilliant, statement is due tomorrow and don't want to incur interest, was gonna pay in full."