TalkTalk experienced a power failure at one of its sites that affected services yesterday (July 20th).
According to the provider, an "unexpected power outage" impacted some of its websites, such as Help, My Account and Webmail.
"We'd like to apologise for any inconvenience caused," TalkTalk said.
"We appreciate your patience during this time and would like to assure you that our engineers are working to resolve the issue asap."
Customers turned to Twitter to vent their frustrations at the company, which insisted engineers were "making progress" and "starting to restore some of our services".
"We'd like to thank you for your patience whilst we continue to resolve the issue," TalkTalk added.
This comes after the company revealed that it added 20,000 customers between April and June this year and that more than a million broadband customers are now signed up to its Fixed Low Price Plans.