Quicktime & FCP will not play a video - but disappeared at reboot!

Yesterday I double-clicked on a video I had exported from FCP 2 weeks ago. The Quicktime player appeared BUT clicking PLAY did nothing. I could scroll through the video and see every frame, but it would NOT play. This is not a version or codec problem - nothing has changed since the last time I exported and played this video.

So, I opened FCP with my current project, loaded the video as media, put it on a timeline and it would NOT play. Nor would any media file already in my FCP library play. Scroll through yes, play NO.

I shut down, tried to play the same video file on my laptop (that worked, same version of QT player) and then when I rebooted my desktop (M1 Studio) everything worked normally. So there is some condition lurking in the software that can cause this problem. It would be great if Apple could find it and eliminate it. But reporting this problem is getting lost in the problem of "older codecs, not supported."