Frances, a few basic questions:
Do you have a full, recent backup?
Are you using POP or IMAP mail?
One possible solution (second post in thread):
http://discussions.apple.com/thread.jspa?m...06Frances, when you go to user->Library->Mail->(choose one of your email account folders)->Inbox, what do you see? You should see a great long list of numbered files with .emlx extensions and if you click once on one of those in the Finder, should see a preview, albeit postage-stamp size, of the email itself. Clicking on it, as Jim says, should make it open in Mail. Most of the emails in my Inbox are 4-12KB in size, if that helps. If they in fact are completely gone - ie: no message content whatsoever, then your only recourse will likely be a backup.
BTW - when I've move computers (which I do whenever I'm going on a trip with my MBP), I just drag and drop the mail folders from the Mac Pro into Mail - that's everything, including the .plist files. That works well. Don't use Import or weird things happen and nothing is arranged the way you want it, in my experience. Migration assistant also works well of course, if you've got a brand new 'puter.