It sounds like the "Save web forms" period expired for the users link. What is the "Expiration" period set for your form on the "Save Web Form" section of the Options tab? The default is a week, if you did not change that and they waited a week it would time out.
Once it expires that saved data is deleted from the server and is not retrievable.
This FAQ on the Save web form feature may be useful: http://forums.adobe.com/docs/DOC-4135
Thank you SO much for getting back to me. The thing is, I set the expiration for 28 days. I know that he hasn't let 28 days pass since he first logged-in, because the form was only distributed on January 10th. Is there anything else that might have happened?
Thanks again for your thoughts on this --