The bank's mortgage system crashed on Monday morning, forcing IT staff to take it offline until 8pm, following what the bank described as an input error.
Customers calling the bank's Stockport headquarters were told that their mortgage accounts were unavailable and were asked to call back the next day.
"The woman I spoke to said she had started her shift at 1pm and had not been able to use the systems at all," one customer told Computer Weekly.
The bank said the mortgage system had malfunctioned on Monday morning following an operator error. By lunchtime, IT staff took the decision to reboot the system, which was back online by 8pm. Co-op's banking customers were not affected.