Could you try shutting down and restarting Windows a couple times? Sometimes after an update there is a user identity or permissions gap that can cause mysterious issues with third party software. When Windows does a ''major'' update -- for example, updating Windows 8.1 to Windows 10 -- it may ask you whether you want your data left in place, backed up, or removed. In the "backed up" case you'll find a folder named C:\Windows.old\ on your system, and your Thunderbird data would be in there. '''If you see a folder like that:''' Set Windows to show hidden files and folders. https://support.microsoft.com/en-us/help/14201/windows-show-hidden-files Then you can drill down and look for C:\Windows.old\Users\''username''\AppData\Roaming\Thunderbird If you find some files there, a Thunderbird expert can suggest how best to migrate them back into their normal locations.