Hotmail owner Microsoft said a sign-in problem affecting some users had been resolved - after users worldwide reported they were unable to access emails.
It is unclear what was behind the problem, though people took to social media in their droves to complain about the outage.
Microsoft said in a brief statement: "Some customers may have experienced difficulty signing in to some services.
"The issue has been resolved."
A spokeswoman was unable to provide any further details when asked about the scale of the outage.
Hotmail, which has been rebranded under Microsoft's Outlook banner, is believed to have crashed before midday GMT.
Microsoft's Twitter accounts were flooded with users desperate for information - with some saying their usual business operations were being damaged by the glitch.
The company responded to one user in Ireland: "Sorry for the inconvenience. We know about the issue and are currently working on the resolution. Request you try and log in again after a few hours."
Another user wrote on a Microsoft community site: "I've had repeated issues in the last 15 mins logging into Hotmail -first it said my account doesn't exist and then it says that my account can't be located, and then finally it asks for my password but the page doesn't look legit."
It comes months after Microsoft disclosed details of cyber attacks by a hacking group that exploited a security flaw in its Windows operating system.
The disclosure in November revealed that a group previously linked to the Russian government and US political hacks was behind the attacks.
It said at the time that it was issuing a new "patch" to protect Windows against the newly discovered threat.
Microsoft has not said what might be behind the latest problems.