dropping dissapearing problem

hi when i drop my object disspears or go t out side of screen or another place please help me this my code: and yes i dont own ts code i just editit using System.Collections; using System.Collections.Generic; using UnityEngine;

 public class draganddrop : MonoBehaviour
 {
     private GameObject startparent;
     public GameObject canvas;
     private bool isdraging = false;
     private Vector2 startposition;
     private bool isoverdropzone1 = false;
     public GameObject dropzone1;
     private bool isoverdropzone2 = false;
     public GameObject dropzone2;
     public GameObject gauge1;
     public GameObject gauge2;
     public GameObject right1;
     public GameObject right2;
     public GameObject left1;
     public GameObject left2;
     public GameObject center1;
     public GameObject center2;
     private bool isovergauge1 = false;
     private bool isovergauge2 = false;
     private bool isoverright1 = false;
     private bool isoverright2 = false;
     private bool isoverleft1 = false;
     private bool isoverleft2 = false;
     private bool isovercenter1 = false;
     private bool isovercenter2 = false;
     private void awake()
     {
         canvas = GameObject.Find("canvas");
 
     }
     // Start is called before the first frame update
     void Start()
     {
         
     }
 
     // Update is called once per frame
     void Update()
     {
         if(isdraging)
         {
             transform.position = new Vector2(Input.mousePosition.x, Input.mousePosition.y);
             transform.SetParent(canvas.transform, true);
         }
     }
     public void begindrag()
     {
         startparent = transform.parent.gameObject;
         startposition=transform.position;
         isdraging = true;
     }
     public void OnCollisionEnter2D(Collision2D collision)
     {
         isoverdropzone1 = true;
         isoverdropzone2 = true;
         isovergauge1 = true;
         isovergauge2 = true;
         isoverright1 = true;
         isoverright2 = true;
         isoverleft1 = true;
         isoverleft2 = true;
         isovercenter1 = true;
         isovercenter2 = true;
         dropzone1 = collision.gameObject;
         dropzone2 = collision.gameObject;
         gauge1 = collision.gameObject;
         gauge2 = collision.gameObject;
         right1 = collision.gameObject;
         right2 = collision.gameObject;
         left1 = collision.gameObject;
         left2 = collision.gameObject;
         center1 = collision.gameObject;
         center2 = collision.gameObject;
     }
     private void onCollisionExit2D(Collision collision)
     {
         isoverdropzone1 = false;
         isoverdropzone2 = false;
         isovergauge1= false;
         isovergauge2 = false;
         isoverright1 = false;
         isoverright2 = false;
         isoverleft1 = false;
         isoverleft2 = false;
         isovercenter1 = false;
         isovercenter2 = false;
         dropzone1 = null;
         dropzone2 = null;
         gauge1 = null;
         gauge2 = null;
         right1 = null;
         right2 = null;
         left1 = null;
         center1 = null;
         center2 = null;
     }
     public void enddrag()
     {
         isdraging = false;
         if (isoverdropzone1)
         {
             transform.SetParent(dropzone1.transform, false);
 
         }
         else if (isoverdropzone2)
         {
             transform.SetParent(dropzone2.transform, false);
         }
         else if (isovergauge1)
         {
             transform.SetParent(gauge1.transform, false);
         }
         else if (isovergauge2)
         {
             transform.SetParent(gauge2.transform, false);
         }
         else if (isoverright1)
         {
             transform.SetParent(right1.transform, false);
         }
         else if (isoverright2)
         {
             transform.SetParent(right2.transform, false);
         }
         else if (isoverleft1)
         {
             transform.SetParent(left1.transform, false);
         }
         else if (isoverleft2)
         {
             transform.SetParent(left2.transform, false);
         }
         else if (isovercenter1)
         {
             transform.SetParent(center1.transform, false);
         }
         else if (isovercenter2)
         {
             transform.SetParent(center2.transform, false);
         }
         else
         {
             transform.position = startposition;
             transform.SetParent(startparent.transform, false);
         }
     }
 
 }