Scripts stop working after closing Unity?

Sorry, if this is a silly question but I'm new to Unity and scripting. I followed a tutorial to make an interactive game object and it works fine until I close Unity/load up a different scene. The code looks like this:

 using UnityEngine;
 using System.Collections;
 
 public class objectSelect : MonoBehaviour {
     public RaycastHit hit;
 
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void LateUpdate () {
         Ray ray = Camera.main.ScreenPointToRay (new Vector3 (Screen.width / 2, Screen.height / 2, 0));
         if (Physics.Raycast (ray, out hit, 3)) {
                         if (hit.collider.gameObject.GetComponent<highlightInteract> () != null) {
                                 hit.collider.gameObject.GetComponent<highlightInteract> ().OnLookEnter ();

                         
     }
     }
 }
 }

thanks in advance