Alright, if you just mess your desktop up, I have a tip for you to reset it to the beginning state.
Firstly, fire up the terminal (Konsole or Yakuake, or your favourite terminal) and type down these commands:
kquitapp plasma' will terminate the plasma session and your desktop will become dark screen. Make sure that terminal window is still opened.
rm ~/.kde4/share/config/plasma-appletsrc' will remove desktop setting file.
plasma' will restart your plasma session and a new setting file will be created. Then your desktop will return to beginning state.
See you next time...