The company's online versions of TurboTax, QuickBooks, Quicken and QuickBase were brought back online Thursday. The Web applications, used mostly by small businesses, had been down since Tuesday night.
Intuit President and Chief Executive Brad Smith apologized to customers for the outage, saying, "there is simply no excuse for having such a negative impact on you."
"I deeply apologize for the pain we have caused those of you affected by this week’s outage," Smith said on the company's blog. "We hold ourselves to the highest standards in dependability and customer service, and over the past two days, we have failed to live up to those expectations."
Smith said the company would work with customers affected by the outage and, where it can, "make things right." The CEO didn't elaborate.
The service disruption occurred during a routine maintenance procedure when an accidental, but severe, power failure affected Intuit's primary and backup systems, bringing down a number of Intuit's Web sites and services.
The company had no evidence of foul play, but said the cause of the outage was still under investigation. Intuit did not believe there was any damage or loss to customer data.
Intuit reported in May that revenue from small businesses grew 13% in its fiscal third quarter ended April 30. The company said the number of customers on its Web sites offering services, such as payroll, employee management and financial services, grew by 80% year to year to more than 300,000.
Also last month, the company announced that it would buy Medfusion for $91 million in cash. Intuit planned to combine its business and financial management products for small and midsize firms with Medfusion's online patient-to-provider communications solutions.