I hid the menu bar in virtual box, how to show it again?

You can access the menu while the VM is running by Right Control + Home (by default). It will pop up as a contextual menu through which you will be able to reactivate the display of the menu.

Open virtual machine's settings, then User Interface page. Check this box:

Screenshot of VM settings window on User Interface page, with an arrow pointing to a checkbox on the top toolbar

Temporary removing of %USERPROFILE%/.VirtualBox folder with

mv %USERPROFILE%/.VirtualBox %USERPROFILE%/.VirtualBox_old

and starting a virtualbox from scratch allowed to solve the issue with the help of

hotkeys(Host+C & Host+Home, where Host=Right Ctrl)

from this tooltip