Enabled Behaviours are Updated, disabled Behaviours are not.
This is shown as the small checkbox in the inspector of the behaviour.
#pragma strict
// Required when Using UI elements.
public class Example {
    public var pauseMenu;
    public function Start() {
        //Enables the pause menu UI.
        pauseMenu.enabled = true;
    }
}
using UnityEngine; using System.Collections; using UnityEngine.UI; // Required when Using UI elements.
public class Example : MonoBehaviour { public Image pauseMenu;
public void Start() { //Enables the pause menu UI. pauseMenu.enabled = true; } }