I just updated yesterday (version 43720 – it’s been a couple of months since I updated). My KDE desktop is black and it takes a long time from logging in to get past the splash screen. Since I had Firefox open when I shut down last, that’s the only app open or that I have GUI access to.
Same thing happend to me with a recent update last week.
I resolved the issue with rm ~/.config/plasma-org.kde.plasma.desktop-appletsrc
Some of the widgets no longer work and are missing but otherwise functional.