In KDE 4, you can do:
    killall plasma-desktop #to stop it
    kstart plasma-desktop #to restart it

In KDE 5 use:
    killall plasmashell #to stop it
    kstart plasmashell #to restart it

In KDE > 5.10 use:
    kquitapp5 plasmashell
    kstart5 plasmashell