Frustrated customers of HSBC faced difficulty accessing online and mobile services owing to two separate IT glitches.
The first occurred just before 20:00 GMT on Thursday, and was fixed about three hours later.
The bank's Twitter staff joked that "the elves had just got out in our server rooms and run amok".
But a second fault then struck overnight, leaving some customers with intermittent access problems on Friday.
HSBC apologised and said that services should now be available again.
The two glitches were unconnected, but had the same effect - making it difficult for some customers to access their accounts. Customers of First Direct, owned by HSBC, were also hit.
After the initial fault was fixed, the company wrote on Twitter, in reply to one customer: "The elves had just got out in our server rooms and run amok, however we've put them all back in their pen now, and everything is sorted."
Many people would be checking balances at this time of year, and Friday would be payday for some customers.
ATMs and branch services were still working as normal.
Some customers would have received a text urging them to clear any overdraft, dipped into during the day, by midnight to avoid any charges.
The fact that some would not have been able to access their account to clear their overdraft could lead to charges, but the bank said these would be refunded automatically.