Tens of thousands of Gmail users found themselves locked out of their accounts Sunday, a glitch Google engineers were still struggling to fix and fully understand Monday. 

Initial reports suggested as many as half a million accounts were compromised, the e-mails collected over the years potentially permanently erased. Google acknowledged the issue at 3:09 p.m. EST Sunday; by Monday afternoon the company lowered the number of affected users to just 0.02% of accounts -- a small number that nonetheless translates into a large number accounts. 

Google has not said exactly how many users its e-mail service has, only that Gmail has "hundreds of millions" of users around the world. That would mean that tens of thousands -- 20,000 out of every 100 million -- of users were affected.

A Google spokesman told FoxNews.com Monday afternoon that all users would have their service back by the end of the day -- but it would be premature to speculate whether there would be any permanent data loss.

And as of 12 p.m. EST Monday, the company was still working to fully resolve the issue.

"We've restored access for 1/3 of those users already," the Google spokesman said, "and engineers are at work fixing it now. Every hour x percent more are restored."

"The remaining 0.013% of accounts are being restored on an ongoing basis, and we expect the issue to be resolved for everyone within 12 hours," the company wrote on its Apps Status Dashboard

Google engineers continue to study the service outage, and will post a complete report within 48 hour or so, the spokesman said, though he could confirm that the problem was "an internal one."

Meanwhile, locked-out and livid users have taken to the company's user forums in their quest for help.

"Yesterday I had exactly the same issues reported here: deletion of ALL mails, archives, labels, filters, pop/imap settings for alternative e-mail addresses, blackberry. It looks like a brand new Gmail account with 7.5GB storage space as opposed to the 25GB I paid for," one user wrote. 

"Google support help please!" wrote another desperate Gmail user. "I cannot access my account, there is really valuable information there for me, this account have been my principal account for 4 years, I am really afraid to think about how much data I lost."

Alex Chitu, whose blog charts the Google's cloud computing initiatives, told the BBC that such outages are rare. But tech site Engadget suggested that the event should serve as a wake-up call for users: Even services that work in the "cloud" -- meaning those that are accessible only online, through a browser -- can potentially fail. This latest incident underscores the need to back up files and data regularly.