I have a kubuntu 25.04 laptop. I updated to 6.4.2. There are two users on this laptop. My user can not log in properly to Plasma after the update. After submitting password, a spinning cogs shows for about 10 seconds, and the a black screen with a mouse pointer (which works). Alt F2 launches the runner, and I can start Dolphin, system settings … but no desktop, no panels.
the other user is not active. I used it to log in, and it worked, it went to the welcome screen.
Using wayland (the default) or X11 makes no difference.
Probably I would like to reset settings, but not sure how.
Since you have krunner, you can try reloading plasmashell from there:
plasmashell --replace
Sometimes this fixes whatever was wrong, and it all works, or run it in Konsole and see if there are errors which might point to a cause.
Segmentation fault (core dumped) 
I created a new user and deleted the original user, this is only a testing laptop and searching for settings reset is pretty convincing there is no easy way to restore plasma settings to defaults.
for anyone who comes along later with a similar problem, I regret not trying this first:
https://discuss.kde.org/t/reset-all-plasma-settings-at-once-and-start-tiling-feature-on-command-line/9503