Backups are the answer to problems like this. If you have a recent backup of your Thunderbird profile on a separate drive, or if the repair person was able to recover the profile information from the damaged drive, then you are in luck. Messages that you have already downloaded have been stored inside your profile. This Support Forum has many articles on how to restore those messages, depending on your circumstances If you have an IMAP mail account, like gmail, your old messages may still be on their servers, unless you specifically deleted them. A new installation of TB, configured properly, can still access those accounts and view any messages remaining on the server. Backup! Backup! Backup!