I'm assiming you are using a Windows OS.

If you have not made a recent backup then do one now in the hope that everything is still intact. Access Profile folders. If using Windows OS and everything is in default location, it should be here: C:// Users /Windows User Account/Appdata/Roaming/Thunderbird Copy the 'Thunderbird' folder to act as backup and paste on external hardrive or usb of suitable size.

Access where Program is stored: Do you see 'Mozilla Thunderbird' in this directory?

   C:\Program Files\Mozilla Thunderbird\ 

Do you see 'Mozilla Thunderbird' in this directory?

   C:\Program Files (x86) \Mozilla Thunderbird\ 

Delete the 'Mozilla Thunderbird' folder. (from both places if necessary) Empty the 'Recycle Bin' Please tell the forum where 'Mozilla Thunderbird' folder was found.

Not sure what version you were using before, but if it was 60.9.0 then you could download that version again from here: http://ftp.mozilla.org/pub/thunderbird/releases/60.9.0/win32/ Select the language you want eg: en GB = English British then download the .exe file. Run and install. Start Thunderbird.

If you want to update to 68.1.0 - download Windows 32 bit option and install from here: https://www.thunderbird.net/en-US/thunderbird/all/

note many addons may not work, so check addons you use to see if they are offering one that works with version 68. Some people are experiencing some issues. Telling the forum where the 'Mozilla Thunderbird' folder was found will also tell us whether it was trying to install Windows 32 bit version or Windows 64 bit version.

re : Dialog box says new version installed (I didn't install one) This seems to have occured as an auto update.

Prior to update were you using Windows 32bit 60.9.0 version ?