The Virtualbox Menu bar disappears if the virtual machine is running in “scale mode”.

Resizing the machine’s window: The virtual machine’s window can be resized when it is running. In that case, one of three things will happen:


– If you have “scale mode” enabled, then the virtual machine’s screen will be scaled to the size of the window. This can be useful if you have many machines running and want to have a look at one of them while it is running in the background. Alternatively, it might be useful to enlarge a window if the VM’s output screen is very small, for example because you are running an old operating system in it.

– To enable scale mode, press the host key + C, or select “Scale mode” from the “Machine” menu in the VM window. To leave scale mode, press the host key + C again.

– The aspect ratio of the guest screen is preserved when resizing the window. To ignore the aspect ratio, press Shift during the resize operation.

How to Define or Set the Host Key

Close all open virtual machines.
Go to Virtualbox File>>Preferences menu.
Click Input button.
Click reset icon.
Click Host Key area and set new host key “Alt Gr + Right Ctrl”
Restart VM press “Alt Gr + Right Ctrl” .
Now VM menu is showing.

References
http://askubuntu.com/questions/59103/why-virtualboxs-menu-has-disappeared
http://www.virtualbox.org/manual/ch01.html#intro-resize-window
http://ubuntuforums.org/showthread.php?t=198027

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