Power steering is dead.. help please?

My steering got VERY heavy this afternoon. I stopped to check the fluid and it was bone dry. I bought a quart of fluid and started filling the pump. It took about half a quart. Once I started the car, I started topping it off. It took the whole quart and then I realized there was a large puddle of fluid on the ground. The leak (or gaping hole) is hidden from view and the pump is not visably leaking. Where should I start diagnosing it? Thanks in advance.