Here are some OSX commands I need to use from time to time when the top menus or dock disappear. It's often triggered by taking an app full-screen and then back again, or by connecting an external display.

The Menubar crashed/refuses to be clickable:

killall -KILL SystemUIServer

The Finder crashed:

killall -KILL Finder

(you can also restart the Finder by option (=alt) + rightclicking the Finder icon in the Dock and selecting Relaunch. It doesn't always work though.)

The Dock crashed:

killall -KILL Dock

Spaces crashed:

killall -KILL Dock

(OP: Mac: Restarting the Mac OS X Dock, Finder, Spaces or Menubar | Stefan Ernst)