There is a known issue that can happen if AutoCAD is installed while logged in as "user1", and then when "user2" starts up AutoCAD, the menus are not present.
But you say that "sometimes my menu disappear", indicating that is has happened more than once, which also means that you've fixed it and it happened again..?
So was it broken since the beginning, or did this just start one day?
Have you ran a repair on the install?
Have you tried to manually reload the stock OOTB menu (cuix file)?