Microsoft's consumer services went down for several hours, but has been "resolved", according to the company.
Users took to Twitter to complain about not being able to log in to their email accounts at Hotmail and Outlook.
A map on DownDetector, where users can log they have problems using the service, showed that many of the complaints were coming from the UK.
Outlook and Hotmail weren't the only services affected by the outage; many people on Twitter also complained of not being able to access their XBox Live accounts.
A Microsoft spokesperson said: "Some customers may have experienced difficulty signing in to some services. The issue has been resolved."