Sometimes, especially if there is an antivirus or firewall intercepting it, an update causes Thunderbird to lose pointer to profile, so it starts a fresh one. That is, your data may still be there, but in an older profile. Profiles are stored in c:\users\<yourid>\appdata\roaming\thunderbird\profiles folder. One way to review that (other than with windows file explorer), is to click help>troubleshootinginformation, scroll down to 'profiles' and click 'about:profiles.' The next pane should show available profiles. I don't know if you can just select and go with another profile, or whether you need to extract data, but look first and see what's there.