Okay, what happens if you use a desktop other than KDE? On Red Hat, for instance, you can select that from the menus at the bottom of the login-window. Does this enable you to run at all?
If it does, then perhaps you can re-install the packages for KDE.
Another thing to look at are any /var/log/messages
The first step, basically, is to try to get something
that you can base either a diagnosis or a solution on. Trying to use "something besides KDE," for instance, will tell you whether the problem is specific to KDE or something deeper. (For example, X-windows.)
Another clue might be ... what changed?
Let's assume for the sake of simplicity that you weren't, in fact, hit by a virus or a stray cosmic-ray ... rather, that something did
change, and that it probably happened right before the system started to go south. (That's not nearly as glamorous a theory but it's a good bit more probable.)
Carefully test to be sure that other user-IDs are
affected. All of the user's KDE settings are in a hidden directory named .kde
and you can
recover sometimes by deleting or renaming it.
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"Don't know where to get help?" Right here!