I'm using 12.04, classic desktop without effects. Once the indicator-applet-complete (right part of the top panel) had disappeared. I found how to fix it with Alt–Win-RightMouseClick on the panel, then "Add to the Panel", where choose "Indicator Applet Complete" to add.
Now, the left part of the top panel (Applications Menu) has disappeared! If I do Alt–Win-RightMouseClick on the panel, I can add only individual menu sections like Internet, Office, Settings etc.
How to get back the full menu?
Best Answer
Answer is: add to the panel the Menu Bar option!