Frog disappears after 4 seconds of pressing the spacebar and I can't seem to find out what's wrong

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

    //Prevent bird from 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 game is not started and the player presses the space bar...
    if (gameStarted == false && Input.GetKeyUp (KeyCode.Space)) {
        StartGame();
    }

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

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

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

            //...then reload the currently loaded scene
            SceneManager.LoadScene(SceneManager.GetActiveScene().buildIndex);

        }
    }
}

private void StartGame() {
    //Set the game state
    gameStarted = true;

    //Remove the start text
    gameStateText.color = Color.clear;

    //Allow player movement
    playerMovement.enabled = true;

    //Allow bird movement
    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);
}

private Animator playerAnimator;
private float moveHorizonatal;
private float moveVertical;
private Vector3 movement;
private float turningSpeed = 20f;
private Rigidbody playerRigidbody;

// Use this for initialization
void Start () {

    //gather components from the player gameObject
    playerAnimator = GetComponent<Animator> ();
    playerRigidbody = GetComponent<Rigidbody> ();

}

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

    //gathering input from Keyboard
    moveHorizonatal = Input.GetAxisRaw ("Horizontal");
    moveVertical = Input.GetAxisRaw ("Vertical");

    movement = new Vector3 (moveHorizonatal, 0.0f, moveVertical);

}

void FixedUpdate () {

    // If the Player's movement Vector does not equal 0...
    if (movement != Vector3.zero) {

        //...then create a target rotation based on movement Vector...
        Quaternion targetRotation = Quaternion.LookRotation(movement, Vector3.up);

        //...and create another rotation that movesd from the current rotation to the target rotation...
        Quaternion newRotation = Quaternion.Lerp(playerRigidbody.rotation, targetRotation,turningSpeed * Time.deltaTime);

        //...and change the player's rotation to the new incremental rotation
        playerRigidbody.MoveRotation(newRotation);

        //...then play the jump animation
        playerAnimator.SetFloat ("Speed", 3f);
    } else {
        //Otherwise do not play jump animation
        playerAnimator.SetFloat ("Speed", 0f);
    }

}