Rick has said its a little sloppy to hook up the game over canvas as he had because if you are in different levels you would have to do it manually.
I wrote a bit of code that should work to auto hook things up.
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
public class DeathHandler : MonoBehaviour
{
GameObject gameOverCanvas;
void Start()
{
gameOverCanvas = GameObject.FindGameObjectWithTag("GameOverUI");
gameOverCanvas.GetComponent<Canvas>().enabled = false;
}
}
essentially instead of making a canvas object i made a gameobject, used the findgameobject with tag function and tagged my game over ui with it. then I used getcomponent to access the canvas part of it and set that to false.