Just upgraded to Fedora 33 and plasma gets stuck here:
https://imgur.com/bofQSaK

Plasma shortcuts still work so I can open terminals and applications but they just don't have Window manager stuff. When I close a window and quit an application, it stays on screen, it doesn't get un-drawn.
Theres no background or anything of course.

Where is the first place I should be looking to find out what happened.

Fedora 33 upgrade did warn me about an incompatibility with libcroco (which is to do with CSS and Gnome?) but thats all.

I suggest checking you have a complete upgrade, and report this to Fedora as well, the packaging is done by distributions.

Did upgrade through Software app.

For plasma, I tried moving .local, and .config out of the way,
deleted .lock files from .local

All is restored now but no diff.

[Edit]
I did an update and it was 300MB... Plasma works again after reboot.

Thanks!!