I don't think you folks understand that I can and do still play YouTube videos -- if they are uploads or favorites on my channel (even just now added) or if I "share" them on Facebook and play them there or embed them on some other site.
I don't understand why that should happen because of some issue with my video-playing software, browser, operating system, etc.

I did post my Q in YouTube's "Help Forum," and I've found that a bunch of other people have recently encountered the same glitch. Some of them have tried all the suggested "fixes" like reinstalling a flash player update, but it hasn't worked. And no one has explained why you should be able to go to YouTube, "add" a video to your "favorites" list and then play it on your channel but not be able to play the same video just by going to YouTube and clicking on it.