The Problem
The ubermenu.css file (basic.css in UberMenu 2), which contains all of UberMenu’s structural styles (the code that makes the menu an mega menu), has been disabled.
The Solution
UberMenu 3
Re-enable the ubermenu.css in the UberMenu Control Panel > General Settings > Assets > Load UberMenu Core Layout
UberMenu 2
Re-enable the basic.css in the UberMenu Control Panel > Advanced Settings > Include basic.css