Musescore 4 Suddenly Closed, Then Stopped Loading

I have been using MuseScore 4.0.2 with Hub on a Win 10 desktop without any problems. Suddenly, the program closed.

It started up again fine, using Hub. But after using it for about an hour, it suddenly closed again. After that, when I tried to start it, I'd get the loading screen for a few seconds, and then it disappears.

I deleted Hub and MS 4, then installed a fresh version of MS 4 only. Restarted my computer. I went to Programs, and clicked to open MS 4. Same thing happens: the loading screen appears for about 5 seconds, then disappears, and the program hasn't loaded.

I have a score I've spent a lot of time working on in MS 4 and I know I can't open it in MS 3. Help!

Thanks.
David