When the Men Bar disappears,
press the {Alt Key}. Your 'Menu Bar' should appear at the top of your screen.
To keep the Menu Bar on the screen, Point & Click the {View} Menu.
It opens a 'Drop Down' Window.
In the Drop Down Window; point at { Toolbars }.
In the side Window point & click "Menu Bar".
Why the Menu Bar likes to disappear, I don't know.