Trying to Re-Instantiate the Zombie(main character) again when replay button is clicked, but unable to

I am making Replay logic for my game, where when I click replay I got to the Main Page. The problem I am facing is that after clicking Play on the game after coming from Replay, the Zombie character in my game is not showing up. The game is running without the player. I am posting the script, check the Replay function which is attached to Replay button in the game.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Assertions;

public class GameManager : MonoBehaviour
{
public static GameManager instance = null;

private bool playerActive = false;
private bool gameOver = false;
private bool gameStarted = false;
private GameObject newZombie;

[SerializeField] private GameObject mainMenu; //contains main menu content
[SerializeField] private GameObject endGame; //contains game over content

[SerializeField] private GameObject zombie;

public bool PlayerActive{
    get{
        return playerActive;
    }
}

public bool GameOver{
    get{
        return gameOver;
    }
}

public bool GameStarted{
    get{
        return gameStarted;
    }
}

void Awake()
{
    if(instance == null){
        instance = this;
    }else if(instance != this){
        Destroy(gameObject);
    }

    Assert.IsNotNull(mainMenu);
    Assert.IsNotNull(endGame);

    DontDestroyOnLoad(gameObject);
}

// Start is called before the first frame update
void Start()
{
    endGame.SetActive(false);
    mainMenu.SetActive(true);      
}

// Update is called once per frame
void Update()
{
   
}

public void PlayerCollided()
{
    gameOver = true;
    endGame.SetActive(true);
    mainMenu.SetActive(false);
    DontDestroyOnLoad(gameObject);
   

}

public void PlayerStartedGame()
{
    playerActive = true;
}

public void EnterGame()
{
    endGame.SetActive(false);
    mainMenu.SetActive(false);
    gameStarted = true;
}

public void Replay()
{
   
    endGame.SetActive(false);
    mainMenu.SetActive(true);
    gameOver = false;
    newZombie = Instantiate(zombie) as GameObject;
   
}