Capital One says an issue with a third-party vendor that left some customers unable to access their accounts has been fixed.
Customers began reporting issues on Thursday as the firm struggled with payment processing and deposits for some of its consumer, small business and commercial clients.
The problems continued into Friday, with Capital One on social media blaming a "technical issue" with a vendor.
According to CNN, the vendor is FIS Global, which says that a power outage knocked systems offline.
Capital One finally confirmed that services had been restored on Sunday morning, telling customers: "We sincerely apologise for how frustrating this experience has been."
It was a bad week for Capital One, which was also sued by the CFPB, accused of cheating millions of consumers out of more than $2 billion in interest.