Sorry for the trouble. I found that just pointing the reader file to desktop opend in desktop.
It sounds as if your default program for Reader files is Tableau Reader and no longer Tableau Desktop. That may or may not be a bad thing. If you open Desktop, you can then open the Reader files or you could drag and drop the Reader files onto a Tableau Desktop executable icon (if on your desktop).
To associate .TWBX files to open with Tableau Desktop and not Tableau Reader, take a look at the attachment.
Thanks for the comprehensive reply. David