Yea, this is more a general QT/KDE question I guess.
I had to reinstall my EndeavourOS machine. Haven’t done much besides reinstalling software and some playing around with the customization options in KDE control center.
Now I want to use KMymoney again. Tried both, the official version and the AppImage from development: there is no menu bar shown in the app.
Any ideas how to get the menu back?
Whatever it is, it only affects KMM but not normal KDE apps
Have you tried using ctrl-m? This toggles the menu bar on many to most KDE applications.
A solution could be to reset all settings to factory default. This can be achieved by stopping KMyMoney and removing (I would rename them with an extension .ori or similar to be able to compare them later on) the following two files:
~/.config/kmymone/kmyoneyrc~/.local/share/kxmlgui5/kmymoney/kmymoneyui.rcThe latter one might not exist which is OK. The former one will be recreated once you start KMyMoney. Compare it against your original version and see if you find something pointing in the direction of the missing menu bar. If not sure, ask here.
Once done you can restore the original files. Please make sure that KMyMoney is not running at the same time.
This happened to me after reinstalling but it was keepassxc with the missing menu bar and not following the system theme, installing plasma5-integration fixed it.
this does nothing in KMM 
Good guess!
the ~/.config/kmymone/kmyoneyrc file had this:
MenuBar=Disabled
removing it fixed the issue.
Still wonder what caused this?
I use the same OS and software on three machines and never had the menu bar disabled.
Is now fixed in master. Option is available in menu and Ctrl+M is working as mentioned by @claydoh.
and it’s ctrl+shift+m if you are in konsole for some reason.
That is because konsole treats Ctrl+M as a carriage return for the CLI.