Have you paid for the dress in full or did you only have a deposit on it? If there was an amount outstanding was there a specific time or date when this had to be paid? Were you supposed to go for any fittings and missed them? If there was money owing and they tried to contact you with no response they may have no option but to sell it if they had a buyer.
I hope it all works out all the same.
you should tell them you are going to post their name on this forum and several hundred others.i've done this with travel issues on trip advisor and the like and it works a treat.they could at least offer you a more expensive dress for the same price as they would probably still not be out of pocket as what you paid retail would still probably cover a much more expensive dress wholesale price.