lost messages when upgrading to eudora 5.1

My father recently upgraded to Eudora 5.1 and as a result has seemingly lost all his previous email messages, which is a major disappointment to him. Here is his heartbreakng story:

" What has happened to me is that I foolishly downloaded a new copy of my Eudora 5.1. I've upgraded my version before, but this time it was the same version, and when I got up again, I found all my old mailboxes and address book that had been on my old d: drive appear to be gone! All my old saved mail, and all my old addresses are either hidden where I don't recognize them, or they are gone. Maybe I'll find them sometime soon. Maybe never. (It doesn't make sense, as they were files on a hard drive that was not on my machine at the time when the new version was installed. Maybe I just don't recognize them. "

Can anyone help with this? Thanks.