I have restarted - several times - I even did a full re-install of software...
To answer your question, @k6ccc, no, nothing had been done between the time Sequencer worked and when I went back a couple of hours later to tweak my sequences.
But I noticed that Windows wanted to update, so I let it... then, when the Sequencer program still didn't work, I updated again... restarted another several times (this took up most of yesterday), and now I'm back and it still won't open.
So I am going to try your suggestion, @MattBrown
... oh another detail: when I first try to open the Control Panel, it too will open its window briefly, then shut down... when I open it the second time, it stays open... this has been happening since the same time Sequencer stopped working... don't know if there is a link.