All I can guess is that thunderbird restarted with a fresh profile or that an antivirus interrupted a process during the upgrade. My suggestion is to dig into profiles to see if an older profile is present. Try these steps: - click help>troubleshootinginformation - scroll down left side to 'profile folder' and click 'open folder' - you are now in the profile. click the 'back' arrow to go back one more folder - you should now be in the ....thunderbird\profiles folder. Do you see more than one profile shown? - if yes, browse each one to see the size and date of the file named abook.sqlite - that is the addressbook. If you find one that was updated recently, that may be it. Look also in the Imapmail folder if you had an iMAP account and in the Mail folder if you had a POP account. - if you see the missing data, report back and we can try next steps.