nemozone

easy way

Because you're in scale mode which causes it to be hidden.

Host + C is the toggle for scale mode (where Host is Right Control by default)

Source : https://askubuntu.com/questions/59103/why-has-virtualboxs-menu-disappeared

Hard way

 sudo vim /home/$USER/.config/VirtualBox/VirtualBox.xml

Edit

<ExtraDataItem name="GUI/Toolbar" value="false"/>
<ExtraDataItem name="GUI/Statusbar" value="false"/>

to

<ExtraDataItem name="GUI/Toolbar" value="true"/>
<ExtraDataItem name="GUI/Statusbar" value="true"/>

ESC :wq!

Party hard!