Microsoft's suite of productivity software was down for thousands of users on Thursday, according to outage tracking website Downdetector.com.
"We're investigating an issue where users may be unable to access multiple Microsoft 365 services," the Windows parent said in a post on X.
Microsoft did not immediately respond to queries on the nature and cause of the outage and when it expects a recovery.
There were more than 20,000 incidents of people reporting issues with Microsoft 365, according to Downdetector, which tracks outages by collating status reports from a number of sources, including user-submitted errors on its platform.
Nearly 4,000 users also reported issues related to Microsoft Teams.