Force a block to drop when being dragged over hole

I'm working on a block game and I am trying to figure out how to have a block drop when it is moved over a missing block. I can't seem to find anything in code that cancels the dragging. Here are the functions for dragging:

Code (csharp):

1. void OnMouseDown()
2.     {
3.         screenPoint = Camera.main.WorldToScreenPoint(gameObject.transform.position);
4.        
5.         offset = gameObject.transform.position - Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, gameObject.transform.position.y, screenPoint.z));
6.     }
7.    
8.     void OnMouseDrag()
9.     {
10.         Vector3 curScreenPoint = new Vector3(Input.mousePosition.x, gameObject.transform.position.y, screenPoint.z);
11.        
12.         Vector3 curPosition = Camera.main.ScreenToWorldPoint(curScreenPoint)+offset;
13.         transform.position = curPosition;
14.        
15.     }