Barclays says it has resolved a glitch that left frustrated customers locked out of their online accounts for hours on Thursday.
The problem began during the morning and hit the Barclays website and telephone banking.
Fed-up customers took to social media to vent their frustration as the outage continued through the day.
A Barclays spokesperson said: "We're very sorry about the technical problems our customers have experienced today."
The spokesperson added: "Everything is now back up and running and we're really grateful for customers bearing with us."
Earlier, the bank had told customers they would still be able to use the Barclays Mobile Banking app to check their balance, as well as make and receive payments.
Customers were told: "You should still be able still make payments to existing payees through mobile banking. However, new payees aren't possible at the moment due to the incident."
It also told them: "You can still use your card to make purchases as normal" and "payments into accounts are unaffected by the issues we've faced today".
But one alarmed customer said money had disappeared from his account.
Paul Girling told the BBC: "A significant payment due into my account has gone missing. It has been confirmed as sent but not received."
Meanwhile Julia from London told the BBC: "The systems have also gone down inside branches with no transactions possible.
"My friend just called from inside one. The manager was trying to placate customers and couldn't advise when they would be back online. "
Others took to Twitter to complain with one claiming to have lost £4,800 and another saying he needed cash to buy food.
Has cost me £4800 in missed profit today and counting. How quickly will Barclays compensate me as cannot even register a complaint over the phone or via internet right now? A very unhappy and frustrated customer for more than 15 years!— SELL OUT SYNC (@SELLOUTSYNC) September 20, 2018
The latest banking problems follow TSB's huge IT meltdown earlier this year, following a botched IT switch which saw customers locked out of their accounts.
Meanwhile customers of online challenger bank Cashplus - which targets people with poor credit histories - were unable to access their accounts, make cash withdrawals, or make or receive payments earlier this week.
The problems have prompted Nicky Morgan, chair of the Treasury Committee, to ask Richard Wagner, chief executive officer of Cashplus, for an explanation and what compensation he is paying victims of the outage.
She said: "While Cashplus may not be regulated as a bank, it claims to offer 'the UK's most seamless banking services experience'. Its customers rightly expect continuous access to their accounts, and it should be held to the same standards of operational resilience as a retail bank.