using UnityEngine;
using System.Collections;

public class Collectable : MonoBehaviour {
  
  bool isCollected = false;

  void Show() {
    this.GetComponent<SpriteRenderer>().enabled = true;
    this.GetComponent<CircleCollider2D>().enabled = true;
    isCollected = false;
  }

  void Hide() {
    this.GetComponent<SpriteRenderer>().enabled = false;
    this.GetComponent<CircleCollider2D>().enabled = false;
  }


  void Collect() {

    isCollected = true;
    Hide();
    GameManager.instance.CollectedCoin();
  }

  void OnTriggerEnter2D(Collider2D other) {
    
    if (other.tag == "Player") {
      Collect();
    }
  }  
}

using UnityEngine;
using System.Collections;

public class PlayerController : MonoBehaviour {

  public static PlayerController instance;

  public float jumpForce = 6f;
  public float runningSpeed = 1.5f;
  public Animator animator;

  private Vector3 startingPosition;
  private Rigidbody2D rigidBody;

  void Awake() {
    instance = this;
    rigidBody = GetComponent<Rigidbody2D>();
    startingPosition = this.transform.position;
  }
    
  public void StartGame() {
    animator.SetBool("isAlive", true);
    this.transform.position = startingPosition;
  }
  
  void Update () {

    if (GameManager.instance.currentGameState == GameState.inGame) 
    {
      if (Input.GetMouseButtonDown(0)) {
        Jump();
      }
      animator.SetBool("isGrounded", isGrounded());
    }
  }

  void FixedUpdate() {

    if (GameManager.instance.currentGameState == GameState.inGame) 
    {
      if (rigidBody.velocity.x < runningSpeed) {
        rigidBody.velocity = new Vector2(runningSpeed, rigidBody.velocity.y);
      }
    }
  }

  void Jump() {
    if (isGrounded()) {
      rigidBody.AddForce(Vector2.up * jumpForce, ForceMode2D.Impulse);
    }
  }

  public LayerMask groundLayer;

  bool isGrounded() {

    if (Physics2D.Raycast(this.transform.position, Vector2.down, 0.2f, groundLayer.value)) {
      return true;
    }
    else {
      return false;
    }
  }
    

  public void Kill() {
    GameManager.instance.GameOver();
    animator.SetBool("isAlive", false);

    //check if highscore save if it is
    if (PlayerPrefs.GetFloat("highscore", 0) < this.GetDistance()) {
      //save new highscore
      PlayerPrefs.SetFloat("highscore", this.GetDistance());
    }
  }


  public float GetDistance() {
    float traveledDistance = Vector2.Distance(new Vector2(startingPosition.x, 0),
                                              new Vector2(this.transform.position.x, 0));
    return traveledDistance;
  }


}

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

public class ViewInGame : MonoBehaviour {

  public Text scoreLabel;
  public Text coinLabel;
  public Text highscoreLabel;


  void Update() {
    if (GameManager.instance.currentGameState == GameState.inGame) {
      scoreLabel.text = PlayerController.instance.GetDistance().ToString("f0");
      coinLabel.text = GameManager.instance.collectedCoins.ToString();
      highscoreLabel.text = PlayerPrefs.GetFloat("highscore", 0).ToString("f0");
    }
  }
}
The code for GameManager.cs:
using UnityEngine;
using System.Collections;

public enum GameState {
  menu,
  inGame,
  gameOver
}

public class GameManager : MonoBehaviour {

  public static GameManager instance;
  public GameState currentGameState = GameState.menu;

  public Canvas menuCanvas;
  public Canvas inGameCanvas;
  public Canvas gameOverCanvas;

  public int collectedCoins = 0;

  void Awake() {
    instance = this;
  }

  void Start() {
    currentGameState = GameState.menu;
  }
  
  //called to start the game
  public void StartGame() {
    PlayerController.instance.StartGame();
    SetGameState(GameState.inGame);
  }
  
  //called when player die
  public void GameOver() {
    SetGameState(GameState.gameOver);
  }

  //called when player decide to go back to the menu
  public void BackToMenu() {
    SetGameState(GameState.menu);
  }

  void SetGameState (GameState newGameState) {
    
    if (newGameState == GameState.menu) {
      //setup Unity scene for menu state
      menuCanvas.enabled = true;
      inGameCanvas.enabled = false;
      gameOverCanvas.enabled = false;
    }
    else if (newGameState == GameState.inGame) {
      //setup Unity scene for inGame state
      menuCanvas.enabled = false;
      inGameCanvas.enabled = true;
      gameOverCanvas.enabled = false;
    }
    else if (newGameState == GameState.gameOver) {
      //setup Unity scene for gameOver state
      menuCanvas.enabled = false;
      inGameCanvas.enabled = false;
      gameOverCanvas.enabled = true;
    }
    
    currentGameState = newGameState;
  }


  void Update() {

    if (Input.GetButtonDown("s")) {
      StartGame();
    }
  }


  public void CollectedCoin() {
    collectedCoins ++;
  }

}