Restart timer not working & causing errors since restart not triggered.

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

public class GameState : MonoBehaviour {

    private bool gameStarted = false;
    [SerializeField]
    private Text gameStateText;
    [SerializeField]
    private GameObject player;
    [SerializeField]
    private BirdMovement birdMovement;
    [SerializeField]
    private FollowCamera followCamera;
    private float restartDelay = 3f;
    private float restartTimer;
    private PlayerMovement playerMovement;
    private PlayerHealth PlayerHealth;
    // Use this for initialization
    void Start () {
        Cursor.visible = false;

        playerMovement = player.GetComponent<PlayerMovement> ();
        PlayerHealth = player.GetComponent<PlayerHealth> ();

        //Prevent the player from moving at the start of the game
        playerMovement.enabled = false;

        //prevent the brid from moving at the start of the game
        birdMovement.enabled = false;

        //prevent the camera from moving at the start of the game
        followCamera.enabled = false;


    }

    // Update is called once per frame
    void Update () {
        //If the game is not started and the player presses the space bar then start
        if(gameStarted == false  && Input.GetKeyUp(KeyCode.Space)) {
            StartGame();
        }
        //If player is no longer alive then end game
        if(PlayerHealth.alive == false) {

            EndGame();

            //increment a timer to count up to restarting...
            restartTimer = restartTimer + Time.deltaTime;

            //and if it reaches the restart delay...
            if (restartTimer >= restartDelay) {

                //then reload the currently loaded level
                SceneManager.LoadScene(SceneManager.GetActiveScene().buildIndex);
            }
        }
    }
    private void StartGame(){

        //set game state
        gameStarted = true;
        //remove start text
        gameStateText.color = Color.clear;
        playerMovement.enabled = true;
        birdMovement.enabled = true;
        followCamera.enabled = true;


    }
    private void EndGame(){
        gameStarted = false;
        gameStateText.color = Color.white;
        gameStateText.text = "Game Over!";

        //remove player from game
        player.SetActive (false);
    }
}```