The menu bar at the top has disapeared
You may have inadvertently toggled Full Screen View - try Ctrl+Shift+J
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If it isn’t a full screen issue then you have probably inadvertently clicked the Menubar icon. On tabbed user interfaces it is at the far left. Click it to reveal the menu again.
Menu revealed
If you don’t have a tabbed interface you can right click a toolbar on which you the menubar icon, select Customise toolbar, under Available commands select Menubar, then under Assigned commands click the icon immediately above the position where you want the menubar icon to go and click the right pointing arrow between the two, click OK.
Click the Menubar icon to toggle menu visibility. Cheers, Al