It appears accounts were reset; Google promises a fix
(NEWSER) - Bad news for less than 0.08%, by Google's count, of Gmail users: Your account may have "vanished into the cloud," Engadget reports. Gmail users started reporting yesterday morning that they'd lost access to their inboxes—and that, by the time some of them were able to log back in, they had lost years' worth of emails, attachments, and chat logs. It appears a bug reset these accounts, and treated their owners as new users when they logged back in. More»