Add a start and an end

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 movving at the start of the game
    playerMovement.enabled = false

        // prevent the bird frrom moving at the start of the game
        birdMovement.enabled = false;


    // prevent the follow camera from moving to its game position
    followCamera.enabled = false;

}

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

    // if the gamme is not started and the player presse the space bar...   
    if (gameStarted == false && Input.GetKeyUp (KeyCode.Space)) {

        // ...Start the Game
        StartGame ();

    }

    // if the player is no longer alive...
    if (playerHealth.alive == false) {

        // ...end the game...
        EndGame ();

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

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

            // ...then reload the currently loaded level...
            Application.LoadLevel (Application.loadedLevel);

        }

    }

}

private void StartGame() {

    // Set the game state
    gameStarted = true;

    // Remove the start text
    GameStateText.Color = Color.clear;

    // Allow the player to move
    playerMovement.enabled = true;

    // Allow the bird to move
    birdMovement.enabled = true;

    // Allow the camera to move
    followCamera.enabled = true;

}

private void EndGame () {

    // Set the game state
    gameStarted = false;

    //Show the Game Over Text
    gameStateText.color = Color.white;
    gameStateText.text = "Game Over";

    //Remove The player From the Game
    player.SetActive (false);
}