I Upgraded from ubuntu 16.04 to 18.04 recently, and when I did the menubar for libreoffice disappeared. I also noticed that the menubar for Google-chrome also disappeared. I know that the update from 16.04 --> 18.04 changed from unity to gnome so perhaps this is an issue where I am just unfamiliar with where to locate the menubar in gnome windows. Any help is appreciated as I've had major loss of productivity since my upgrade.
EDIT: I just checked and it appears that while google-chrome and libreoffice are missing the menu, other applications like pinta have a menu within the application window. See screenshots.
Here is the Pinta screengrab. notice the menu in window.
Here is the libreoffice screengrab. Notice the lack of menu in window.
The themes were messed in my case.
I installed the package
gnome-tweak-tool
to get a handle into the "tweaks" application. Further I changed the appearance theme to one that restores sanity.Further I installed the package
gnome-themes-ubuntu
.