If you downloaded two versions and installed both of them, my guess would be that one was from Microsoft, I suggest going to the search box on task bar and entering 'uninstall' and that should bring up the 'add and remove app.' From there, you should see the installed apps.

Many thanks. Tried that and still no sign of thunderbird. Whilst thunderbird is still showing in task bar nothing in apps. Weird.

Ok. this next suggestion is overkill, but it works. One of my favorite free utilities is 'Everything' from https://www.voidtools.com/ It's tiny, and can search your hard drive in milliseconds. Install it and enter thunderbird.exe and it will find it almost faster than you can remove fingers from keyboard. Post where you found it and we can assist. (and my guess is you will keep Everything forever - it's a jewel.)