Sometimes the the gnome-panel process falls over, resulting in hanging or frozen menu bar or context menu. Particularly annoying is when the main application menu bar panel hangs, resulting in a pretty unusable system.
Luckily there is actually quite a simple fix for this in order to force a restart of the gnome-panel system.
First, open a terminal window by pressing Ctrl+Alt+t
Once up, run:
That’s it. Watch in amazement as the top and bottom menu panels disappear, and then after a brief moment, reappear as the system automatically restarts the shutdown gnome-panel process.