If you change the theme or change the font size, the menu is disabled. When you press and hold Alt, then menu does not open. When you use the keyboard shortcut Alt+F menu also does not open.
To restore it you have to restart the editor.
I use:
Sublime text 3, build 3083, unregistered
Compiz 0.9.12
Unity 7.2.5
Ubuntu 14.04.3 LTS (x86_64; Unity)