Brick Breaker - Ball not bouncing back from the brick.

There is no code making it bounce back, and because it's destroying the brick, there's nothing there to bounce back from. You should just inverse the y velocity of the ball's Rigidbody2D before you destroy the gameobject in the OnCollisionEnter2D function.

EDIT: (Thanks to @fafasefor helping improve this answer!) To do this, just add this line of code to the top of your script public Rigidbody2D ball; then go to the inspector, assign the ball to the to the class. Now just add this line of code ball.velocity = new Vector2 (ball.velocity.x, -ball.velocity.y) to the`OnCollisionEnter2D` function. All this is doing is inverting the balls y velocity to send it back down.

Your class should now look like

  using UnityEngine;
  using System.Collections;
  
  public class Brick : MonoBehaviour {
  
      public int maxHits;
      private int timesHit;
 
      public Rigidbody2D ball;
 
      private LevelManager levelManager;
  
      // Use this for initialization
      void Start () {
          timesHit = 0;
          levelManager = GameObject.FindObjectOfType<LevelManager>();
      }
      
      // Update is called once per frame
      void Update () {
  
      }
  
      void OnCollisionEnter2D(Collision2D col){
          timesHit++;
          if (maxHits == timesHit) {
              ball.velocity = new Vector2 (ball.velocity.x, -ball.velocity.y)
              Destroy (gameObject);
          }
      }
  
          //TODO change this later;
      void SimulateWin() {
          levelManager.LoadNextLevel ();
      }
  }

Note that I changed the ball's velocity before destroying the object, this is very important. If I had destroyed it first, then the velocity would never have been changed.