What exactly is it doing, not doing, and what is expected? Any logs? Error messages? My magic eightball told me to ask again later
Sometimes you can ctrl-alt-F1 to get to VT 1 and hopefully theres a tid bit of information spewed out for you.
Try restoring the default /etc/kde/kdm/* files. Perhaps you inadvertently changed one of those, and set an incompatible setting. I believe they are from the kdebase-workspace package (for KDE 4.x).