Behaviour.enabled

Description

Enabled Behaviours are Updated, disabled Behaviours are not.

This is shown as the small checkbox in the inspector of the behaviour.

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;
    }
}