Hello Helen:
There should be an indication of dead trunk/live trunk, oftne by the color. Your cut should be above the live tissue by an inch or so and at an angle.
The weather and growing conditions conspired against you this time, I would not expect this to occur again. Keep the recovering tree well watered (willows can take a lot without harm). Sandy soils dry out quickly and heavier soils hold moisture longer. Gauge your watering accordingly.