This issue sounds very much like this AskUbuntu question. Can you try their solution?
That is:
Go System settings → Startup and Shutdown → Background Services and find and uncheck Application menus daemon. Log out or reboot and it will turn back.
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OMG THANK YOU 
That fixed it! That finally fixed it! Thank you so much!
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It does mean that there is something wrong: I have the “Application Menus” daemon running on my system, and it doesn’t hide the menus. I think the issue may appear if you enable “global menus” and then disable it but then the daemon gets confused somehow and does not stop hiding the menus. I’ll look into that.
I honestly have no idea how i might have done that? Other than changing minor things like pointer size and my desktop wallpaper I don’t tend to much around with my system settings that much. Or at least stuff like that. Because I don’t want to break anything!
But I appreciate you looking into it!
I ran into this and the problem seems to have been that I added the “Application menu” widget to the title bar (System Settings → Appearance → Window Decorations → Titlebar Buttons). When I removed it the “Ctrl-M” option reappeared as did the workaround of putting the “Show Menubar” button in the toolbar.