Lost Thousands of Emails?

I have used YAHOO! since the 90's. I'm a dedicated YAHOO! user and have never had a problem. However tonight, when I went to search my emails (I've kept copies of sent and received emails going back to 2000) I saw that my archive only goes back to 2005, now. What gives with that? Has anyone else had this problem or know why they would be gone? I'm a very avid computer user and it's nothing obvious ... like me deleting them or something like that.