Countdown not working

Hi, I’m trying to make a Countdown timer with the GUI system. My code is not showing any errors, but the timer is not working. A little help would be appreciated.

using UnityEngine;
using UnityEngine.UI;
using System.Collections;

public class Countdown : MonoBehaviour {

  public int countdownNum = 3;
	public Text CountdownText;

	void Start() {
			CountdownText = GetComponent<Text>();
	}

	IEnumerator CoUpdate() {
		while (countdownNum > 0) {
                    yield return new WaitForSeconds(1f);
			countdownNum = countdownNum - 1;
			CountdownText.text = countdownNum.ToString();
			}
		if (countdownNum <= 0) {
		Destroy(CountdownText);
		}
	}
}

You forgot to start running your coroutine. Place StartCoroutine(CoUpdate()) in Start()