Sounds like you have had an update and it went squiffy. Suggest you uninstall Thunderbird. Exit Thunderbird. Either manually delete by going to C:\'Program Files (x86)' and delete the 'Mozilla Thunderbird' folder Then empty the 'Recycle Bin' OR In search type: 'Programs' and select 'Add or Remove Programs' Select 'Mozilla Thunderbird and click on uninstall. then download and install Thunderbird from the following link. Choose language and OS type. https://www.mozilla.org/en-US/thunderbird/all.html

See https://support.mozilla.org/en-US/questions/1322028