Question
if the leaves have fallen off a phalenopsis, will it survive?  The leaves appeared healthy but, at the base, they apparently had turned black and soft.

Answer
Amy, I'm not optimistic about survival chanes.  This is black rot and it probably affected not only the leaves but the tissues at their base which are necessary to produce new leaves.  It sounds like the potting mix was degraded and, therefore, kept too moist, leading to rot in the tissues nearest the potting mix.  You could unpot it, rinse off the roots and check their health.  You may also have root rot from this degraded potting mix.  If so, the roots also will appear mushy. As a safeguard, it is a good idea to repot an orchid in fresh orchid potting mix in a well drained pot after it goes out of flower.  Often the old potting mix has already degraded or consists of moss at the time of purchase. This retains moisture and is easy to overwater leading to rot.  You could try soaking the plant in a solution of physan 27 and repot it in fresh orchid potting mix.  Chances of success, however, are not likely to be high.