How to make player not movable when paused?

Use the boolean. If player paused the game set bool to true, if player is playing the game bool is false.

 public static bool paused = false;
      void OnMouseDrag() {
             if(paused == false){
              var point = Camera.main.ScreenToWorldPoint(new Vector2(Input.mousePosition.x, 
                  (transform.position.y - Camera.main.transform.position.y)));
              point.z = transform.position.z;
              transform.position = point;
              }
 }
  public void Pause () {
         paused = true;
          Time.timeScale = 0;
          gamePausedPanel.gameObject.SetActive (true);
      }

Put all code into one script to avoid sharing the value with other script.