If you can still start up the System Settings
you could try setting it to start with an empty session and see if that restores your desktop. You'll find the Session Manager under the Advanced
tab. Once you've applied the setting log out, then log back in again.
If that doesn't work the other method you could try is removing your hidden .kde4
folder. You do this before logging in.
Do Alt & Ctrl & F1
to swtich to a terminal.
Log in as yourself.
Once that is done, type exit
to log out then do: Alt & F7
to bring back your graphical login. Now log in as normal.
I'm not sure if this applies to the first method but, with the second method you will
lose any customisations you have done to your desktop (compiz should still work) such as plasmoids on the desktop, custom colour schemes etc. You are in effect resetting your desktop back to default.
** To be on the safe side backup your personal data before taking any action **