There are menu items missing form my System->Preference menu. Going to System->Preferences->Main Menu, the menu items are not disable, but missing entirely. How do you reset the menus back to defaults?
2 Answers
Right click the menu, click edit menus, click the "Revert" button. This is the screen you should see:
If the first answer does not work you could try deleting your ~/.local/share/applications directory and then logout/login. Everything should have reset to the default settings.