In your home directory type "cat .dmrc" and see what it defaults to. For me it's:
If yours is different, edit it to either kde or gnome depending on what you used to use.
Edit: Come to think of it, do you have some sort of autologin? I ask while you should see the kdm login screen no matter which desktop you use.
System -> etc/sysconfig Editor -> Desktop -> Display manager -> Erase your name from the autologin window.
Do a ctrl-alt-backspace to restart X, and see if you got kdm back. If yes, you can select kde from the session menu.
You could also try to set DISPLAYMANAGER="kdm" in /etc/sysconfig/displaymanager and DEFAULT_WM="kde" in /etc/sysconfig/windowmanager. Setting the displaymanager should be enough though to be able to select the session.