GameObjet.find give "Missing"

Hello everyone, am facing a strange problem here, i have a class named "MainCowboy" and game objects that use this class are with tag "cowboy", these objects are a child of prefabs (lvl1, 2, 3, etc..)
now the problem is when i load a level (any) for the first time, Game.Object.FindWithTag works perfectly as expected, however, once i load another level, the variable with type MainCowboy become "Missing (MainCowboy)" thank you EDIT : here is the class that contain the error, i removed all the unnecessary lines just to make it more clearer, the loadLevel() method is called when i press a GUI button in the guiManager class .

 using UnityEngine;
 using System.Collections;
 
 public class GameManger : MonoBehaviour
 {
     
     
     public MainCowboy cowBoy ;
     public GameObject[] prefabs ;
     public GameObject level;
 
     void Start ()
     {
         loadLevel (1);
         cowBoy = GameObject.FindWithTag ("cowboy").GetComponent<MainCowboy> ();
     }
     public void loadLevel (int id)
     {
         Destroy (level);
         level = Instantiate (prefabs [id], transform.position, transform.rotation)as GameObject;
         cowBoy = GameObject.FindWithTag ("cowboy").GetComponent<MainCowboy> ();
     }
 
     
 }