Stange Error, please help...

Hello! I have a strange problem that occurred. I have a script viewing the players in a scene over the network, It was working fine, fully finished on the tutorial. However... as im working on my menus or multiplayer menu then switching to my main menu (scene) I get this error message when i load the scene... "The type or namespace name MonoBehaviour' does not exist in the namespace Photon'. Are you missing an assembly reference?"

Here's the script...

 using UnityEngine;
 using System.Collections;
 
 public class NetworkCharacter : Photon.MonoBehaviour {
 
     Vector3 realPosition = Vector3.zero;
     Quaternion realRotation = Quaternion.identity;
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
         if ( photonView.isMine ) {
         
         } 
         else {
             transform.position = Vector3.Lerp(transform.position, realPosition, 0.1f);
             transform.rotation = Quaternion.Lerp(transform.rotation, realRotation, 0.1f);
         }
     }
 
     public void OnPhotonSerializeView(PhotonStream stream, PhotonMessageInfo info){
 
         if (stream.isWriting) {
             // This is OUR player. We need to send our actual postion to the network.
 
             stream.SendNext(transform.position);
             stream.SendNext(transform.rotation);
         } 
         else {
             // This is someone else's player. We need to recive their postion (as of a few milliseconds ago, and update our version of that player.
 
             realPosition = (Vector3)stream.ReceiveNext();
             realRotation = (Quaternion)stream.ReceiveNext();
         }
     }
 }