This sometimes happens if an antivirus product interrupts the update, causing portions of the profile to become corrupt, at which time Thunderbird asks to start over. if you click help>troubleshootinginformation, scroll down to 'profile folder' and click 'open profile', you may see your accounts are there in the Imapmail or Mail folders. If not, you might click 'profiles' instead of 'profile folder' and click 'about:profiles to browse any other profiles on your computer to locate the accounts and addressbook.