You're mail might not be lost. Try these steps:
1. Open Mail.app
2. Click on File > Import Mailboxes
4. Choose Mail for OSX, click the right arrow (next)
5. Leave "Mailboxes" checked and click the right arrow (next) again
6. Click the right arrow (next) again
7. Navigate to the following directory with the file browser that pops up:
hard drive / users / library / mail /
8. Click choose
That might bring back the mailboxes...