Ball disappears after invoking a method

Hi! I'm trying to make a Pong game as a "challenge", it pretty much works, except I'm having problems when a player scores a point.

The following should happen:

• Game starts, ball in 0,0 with zero velocity

• Wait 3 seconds

• Invoking the SetVelocity method, which sets a starting velocity for the ball

• Ball bouncing of the top and bottom walls, and the paddles

• A player scores a point

• Calls for the Start() method so:

• Set the ball's velocity to 0

• Set the ball's position to 0,0

• Wait 3 seconds

• Invoking the SetVelocity method, which sets...

And this goes on..

But, when after the ball position is set after when a player scores a point, the ball gets to center, it stays there for a little but definitely a lot less than 3 seconds and disappears. And I have no idea why. Any suggestions?

Oh and by the way here's the ball's code: (also tried if not calling the start method, but doing the same, no difference)

 using UnityEngine;
 using System.Collections;
 
 public class Ball : MonoBehaviour
 {
 
     public float speed;
     public ScoreHandler scorehandler;
     public float delay = 3.0f;
 
     void Start ()
     {
         rigidbody2D.velocity = new Vector2 (0f, 0f);
         //Debug.Log ("Setted velocity to 0");
         transform.position = new Vector3 (0.0f, 0.0f, 0.0f);
         //Debug.Log ("Setted position to the middle");
         Invoke ("SetVelocity", delay);
         //Debug.Log ("Invoked SetVelocityMethod");
     }
 
     void Update(){
         //Debug.Log (rigidbody2D.velocity);
     }
 
     void SetVelocity (){
         //Debug.Log ("Setting starting velocity");
         rigidbody2D.velocity = Vector2.one.normalized * speed;
         //Debug.Log ("Setted starting velocity");
     }
 
 
     float hitFactor (Vector2 ballPos, Vector2 racketPos,
                     float racketHeight)
     {
                 
         return (ballPos.y - racketPos.y) / racketHeight;
     }
 
     void OnCollisionEnter2D (Collision2D col)
     {
         //Debug.Log (col.gameObject.name);
         if (col.gameObject.name == "paddleLeft")
         {
             //Debug.Log (col.gameObject.name);
             float y = hitFactor (transform.position,
                               col.transform.position,
                               ((BoxCollider2D)col.collider).size.y);
             
             Vector2 dir = new Vector2 (1, y).normalized;
             
             rigidbody2D.velocity = dir * speed;
         } else if (col.gameObject.name == "paddleRight")
         {
             float y = hitFactor (transform.position,
                               col.transform.position,
                               ((BoxCollider2D)col.collider).size.y);
             
             Vector2 dir = new Vector2 (-1, y).normalized;
             
             rigidbody2D.velocity = dir * speed;
         } else if (col.gameObject.name == "wallRight")
         {
             scorehandler.AddLeft ();
             /*rigidbody2D.velocity = new Vector2(0f,0f);
             transform.position = new Vector3 (0.0f, 0.0f);
             Invoke ("SetVelocity", delay);*/
             Start();
 
 
         } else if (col.gameObject.name == "wallLeft")
         {
             scorehandler.AddRight ();
             /*rigidbody2D.velocity = new Vector2(0f,0f);
             transform.position = new Vector3 (0.0f, 0.0f);
             Invoke ("SetVelocity", delay);*/
             Start();
 
         }
     }
 }