Why is my script not working?

using UnityEngine; using System.Collections;

public class PauseMenu : MonoBehaviour {

 public string mainMenu;
 public bool isPaused;
 public GameObject pauseMenuCanvas;


 // Update is called once per frame
 void Update () 
 {
 if (isPaused) {
         pauseMenuCanvas.SetActive (true);
     } else
         pauseMenuCanvas.SetActive (false);
 }

 if(Input.GetKeyDown(KeyCode.Escape))
 {
 isPaused = !isPaused;
 }

 public void Resume ()
 {
 isPaused = false;
 }

 public void Exit ()
 Application.LoadLevel(mainMenu);

}