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: