Workers around the world were hit when Microsoft Outlook and Team services crashed.
The sites have been affected by outages.
More than 6,000 people in Britain – many working from home – were stopped in their tracks when Outlook wouldn’t open.
UK users started to report problems just after 07.00.
Meanwhile, users in India, Australia, and the United Arab Emirates were knocked by the problem.
Microsoft said it had found that a “wide-area networking (WAN) routing change” caused the issues.
It added: “Some of the customers who had previously reported impact are also reporting recovery.”
WAN allows users access information they need for their job when working at home or travelling for work.
It also gives people online access to banks and shops.
Microsoft said the problem was linked to an internal change to their infrastructure.
The company said:
“We’ve rolled back a network change that we believe is causing impact.
“We’re monitoring the service as the rollback takes effect.”
Jake Moore, a cyber security specialist, said:
“These outages are increasing in volume due to the sheer numbers of more online users and traffic.
“Many people are quick to suspect a cyber-attack but this can often add to the confusion.
“It highlights the importance of these services and the millions of people they serve.
“Businesses must test their infrastructure and have multiple fail safes in place however large the company is.”