Why are scripts not working in the new Unity scene?

Question

Mobile development

In one scene, the scripts work, I created a new one, wrote the script, dragged it to the object, but it does not work.
Wrote the simplest

using UnityEngine;
using System.Collections;

public class Nextlevel : MonoBehaviour {
  void OnMouseDown(){
    Debug.Log ("Test");
  }
}

But, in the old scene, it works, but not in the new one.
What could be the problem?