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.rc
The 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.