OK, so I uploaded my pbix file today and it is working well on line.
Went back to make one or two more tweaks to the pbix desktop and the desktop file then became corrupted. I've been pretty good about saving backups, that is until the last 2 weeks, a time during which I have done considerable work on the desktop file that I would not want to lose.
Logically, there must be some way to download the good pbix file that I uploaded just this morning. Please tell me I am right.