Similar problem here too. Allowed Mail to upgrade messages and then it failed to show the message viewer after it completed.
I quit Mail. Restarted it. The application menu bar appears, but when I click on Window, Message Viewer, the message viewer doesn't appear. I've checked my spaces and can't see the main message viewer anywhere.
If I click on File, New Message, however, a new message window does appear. But if I click on File, New Viewer Window, the spinning beachball appears, Mail hangs, and I have to force quit.
I tried rebooting - that didn't help either.
Good job I have Sparrow Mail installed as a backup client.