Though pretty slick and usable by now, KDE4 is not exactly stable all the time, especially plasma.
ToDo after a desktop crash (found here) - if you are lucky enough to still have a shell:
kstart plasma-desktop (see comment by Casian, thanks!)
I added #!/bin/bash in front and copied the script to /usr/bin to have it at the ready. It's cleaner that just killall plasma-desktop & plasma-desktop, because the system configuration cache is rebuilt first and KDE handles quit and start.
For pre-KDE 4.3 users:
plasma is now plasma-desktop, so, in the code above change plasma-desktop to plasma.