Google's Gmail service was down on Tuesday morning, leaving many users with no access to their email, calendar or contact information.
Users of micro-blogging site Twitter complained that they could not load the site.
One Gmail user wrote on Twitter: "Seriously Gmail...I had some things to do this morning with you. You let me down. Hard."
Others talked of having to revert back to Lotus notes and other email accounts, while offline Gmail users celebrated still having access.
Meanwhile, Microsoft used the opportunity to poach users from its rival. " Need a new email account? Take a look at windowslive.co.uk - awesome!", twittered Mel Carson, Microsoft's advertising community blogger.
Google said that it is working on the problem, and has not yet determined the cause. A statement is due shortly.
A running script of problems Twitter users are having with their Google accounts can be seen here.
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