My box had a little hickup today. Maybe it was just me, if not, here is what I did.
Logging in and starting kde was not possible, the screen stayed black with enabled compositing or showed the kdm background image without compositing. Logging in with a fresh .kde folder was possible. I identified the problem to be a doodoo with the RecentDocuments folder.
So, if you cannot log in today, try moving the RecentDocuments folder. If that helps, you can delete it.
mv RecentDocuments RecentDocuments-bac