My player magically disappears after level restart.

I have a flappy bird type game, it works all fine. But when my player dies and the level restarts, the player is not on the screen anymore.

code:

 using UnityEngine;
 using System.Collections;
 
 public class Player : MonoBehaviour {
 
     // force to add
     public Vector2 Force = new Vector2(0,300);
     
     // Update is called once per frame
     void Update () 
     {
         // if the player hits the space key
         if(Input.GetKeyUp (KeyCode.Space))
         {
             // set velocity to zero
             rigidbody2D.velocity = Vector2.zero;
             // add force
             rigidbody2D.AddForce (Force);
         }
 
         // get screen position
         Vector2 screenPosition = Camera.main.WorldToScreenPoint(transform.position);
 
         // if the screen position is more than screen height or less, restart level
         if (screenPosition.y > Screen.height || screenPosition.y < 0)
         {
             respawn();
         }
     }
 
     // restart level when player dies
     void OnCollisionEnter2D (Collision2D col)
     {
         respawn ();
     }
 
     void respawn()
     {
         Application.LoadLevel (Application.loadedLevel);
     }
 }

pictures

before restart

after restart