Open up the Main Menu (Esc) look for the Edit UI button which on the new UI interface it is Menu (ESC) -> Settings (F10) -> Edit UI. Then look for the 'Mount' element UI box, in the top-left of your screen, where you expect your missing menu to be and click on the 'eye' icon within it to make it visible, you can also click and drag to move it around wherever you like on your screen if desired. In this way you can edit a fair amount of the UI (moving or hiding/showing elements) as you wish to customise it to your like.