One Last GUI Question

So I been working on this script for a little bit now. I finally got it to work right which I'm happy about. I added a little bit to it and theres no errors but I have a problem. See the thing is when I start the game Screen.LockCursor = true which I like. And I added that into the script. And I added when you open the menu Screen.LockCursor = false and that works. But my problem is when I close the menu Screen.LockCursor still is false. Can you help out?

 var GUIon:boolean;
 function Start(){
     GUIon = false;
     Screen.lockCursor = true;
 }
 
 function Update(){
     if(Input.GetButtonDown("Menu"))GUIon=!GUIon;
 }
 function OnGUI(){
 
    if(GUIon){
       Screen.lockCursor = false;
       GUI.Box (Rect (0,0,500,300), "FOV");
 
       if (GUI.Button (Rect (20,20,80,80), "FOV = 70")) {
         Camera.main.fieldOfView = 70;
       }else if (GUI.Button (Rect (100,20,80,80), "FOV = 80")) {
         Camera.main.fieldOfView = 80; 
       }else if (GUI.Button (Rect (180,20,80,80), "FOV = 90")) {
         Camera.main.fieldOfView = 90; 
       }
    }
 }