Microsoft on Monday said its Hotmail problem affected more than 17,000 users, but insisted that the problem has since been resolved.

Starting December 30, Windows Live Hotmail users trying to access their e-mail were met with empty inboxes. The problem affected 17,355 accounts, which saw their sent messages, inbox, and folders disappear.

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“Customers impacted temporarily lost the contents of their mailbox through the course of mailbox load balancing between servers,” Microsoft’s Chris Jones said in a Monday afternoon blog post. “We identified the root cause and restored mail to the impacted accounts as of yesterday evening, January 2nd.”

Jones had few other details, but said Microsoft would “fully investigate the cause and will take steps to prevent this from happening again.”

“We’re very sorry for the inconvenience this may have caused to you, our customers and partners,” he continued.

Over the weekend, Microsoft’s official support forum started showing posts from people who reported that their entire Hotmail accounts were gone. Users were able to log in, but there were no e-mails in their inbox, no sent or deleted messages, and no folders.

One user reported getting an error message upon sign-in on December 31 before seeing a “new” Hotmail account welcome message.

Earlier today, Microsoft said it had restored the e-mails to those who were affected, but encouraged those who might still be affected to provide detailed information about their trouble on Microsoft’s Windows Live Help site.