I recently installed two computers with Debian testing and KDE.
Much to my surprise the menu bar has disappeared in Dolphin, so I am unable to get to View -> Hidden Files.
How can I get the menu bar to show?

On the right is a spanner icon, clicking on it will open a menu. Last option is show menubar.

Alternately, you can press the keyboard shortcut Ctrl + M, which enables you to recover Dolphin should you ever manage to disable the toolbar without resorting to deleting configuration files.