User has an Excel document that was being shared, worked in document for 6-7 hours, glanced back to see that the data entered was now gone.  After checking the time stamp "modified" it showed a date that was a day old, so I had him make a small change and click save, the date modified changed.

How is it possible, if absolutely nobody else is accessing the document, all data entered just disappears for no reason.  Auto-save is disable for this worksheet.

Turns out to be user error.  User had two sessions open and saved the session that did not have the changes. 

Wow.

Thanks to share the reason why the error caused, it will help the others who meet the same issue.