If for whatever reason, your desktop is stuck and neither mouse nor keyboard entries will be accepted anymore there might still be a chance of rescueing the session in case your machine is still reachable from outside via ssh.
Once you have successfully sshd to the machine (this might take a while when the system is overloaded and memory is cramped) issue
# kwin_wayland --replace
This restores the session for me.
If the system is still in an unusable state, maybe
# systemctl restart --user plasma-plasmashell
will help. I didn’t have to go that far yet.