I have a phalaenopsis that is recovering from complete root loss. As long as there are healthy leaves, water can be taken up through them to a lesser extent than with roots (humidity must be increased). This is when you do a sphag and bag rescue. Crown rot and complete leaf loss is harder to overcome. I have heard miracle stories where a phal with crown rot started a new plant off the main shoot. The best treatment as with most things is prevention - once a month application of an antimicrobial agent.