On some rare occasions I find that some parts of the KDE desktop might disappear, like the Start menu or Panel for example. Restarting the KDE desktop is easy in such situations.
Switch to a terminal or open the Run dialog (
Ctrl + F2) or switch to the virtual terminal (
Ctrl + Alt + 1).
If you are using KDE 4.x or Kubuntu 14.04, try:
If you are using KDE 5.x or Kubuntu 16.04, try: