I was trying to find atolboo's helpful how-to to restore the plasma desktop. Being in a hurry I couldn't find it.
First try the following which restarts plasma. Open terminal/konsole:
background will be black
restart it with:
If still no Shutdown button try:
Renew to default desktop in terminal/konsole enter:
Here's another how-to to save your original desktop settings to a backup file first, before running kaptan:http://kubuntuforums.net/forums/index.php?topic=3110033.0