Something went wrong during the install. To be safe, backup the your Thunderbird Profile folder. The default location is: "C:\Users\**windows username**\AppData\Roaming\Thunderbird\Profiles\xxxxxxxx.default" Where the xxxxxxxx is 8 random letters & numbers, and the part after the period is the name of the Profile. Copy, '''do not move''', the folder to another location for safekeeping. You will need to turn on [https://www.howtogeek.com/howto/windows-vista/show-hidden-files-and-folders-in-windows-vista/ Show Hidden Files and Folders] to be able to see these folders. Thunderbird stores all personal data such as messages, address books, calendars and configuration settings in a hard drive folder called the Profile. This folder is separate from the installation of Thunderbird. When Thunderbird is uninstalled, it will leave behind your Thunderbird Profile folder, which will be found and used again when you install Thunderbird again. Despite this, it is a good idea to [https://mzl.la/2lBfidM#w_backing-up-a-profile backup the Profile folder]. Once your Profile folder is backed up, uninstall Thunderbird, then reboot your computer. Since I'm a Windows computer tech, I recommend that you run a Disk Cleanup and a Check Disk now to cleanup and fix file errors. [http://brucejohnson.ca/DiskCleanup.html How to run a "Disk Cleanup" in Windows] [http://brucejohnson.ca/CheckDisk.html How to run a "Check Disk" in Windows] [https://www.mozilla.org/thunderbird/ Download & install the latest version of Thunderbird]. It will probably work now, and automatically use the existing Thunderbird Profile.