I have had this happen as well. As a first step, try creating a brand new user while logged in as root, and then see if that new user will log in normally to KDE. If this is the case, your system is intact, and it is something specific to your current user account that is the source of the problem.
Typically, the behaviour you described is the result of a KDE crash. Since you haven't completed the login yet, I would guess that KDE is attemting to autostart something that crashes and burns, taking KDE with it.
Check your ~/.kde/Autostart directory to see what if anything may be going on there. I would also look at your ~/.xinitrc file for the same sort of thing - you may not HAVE a .xinitrc file, but it is another place where things can be autostarted when you login. Finally, check your ~/.profile file - yet another place where things can be autostarted.
In all cases, assuming the new user I asked you to create initially is working OK, you are hunting for something in your current user account that is getting autostarted with KDE, but isn't working properly. Good luck!