I'm afraid it sounds like your plant is probably pretty well dead. The biggest problem I've seen with plants purchased at the chain stores is that often the managers of the orchid areas don't understand the plant's needs. The major problem is that they are overwatered. Orchids generally need to dry out between waterings, not stand in water.
If you want to try to save it (and of course there is no guarantee), take the plant out of its pot and take all the potting medium off its roots. Let it dry out for about 3 days and then put it in a clear plastic bag (leave a little opening for air). Put the bag in an area that gets light, but no sun. If the plant's going to survive, you should see some white and green roots starting to grow in a couple of weeks.