They might, but they may only be able to give you the value of the last price at which that dress was sold. For example, when I worked at a department store back during my college years, we had someone bring in a comforter. It was a similar situation--it had been in a closet, and was never used. It also still had its tag. We were only able to refund her the clearance price for it.

Some stores also have a limited return time frame. It may be worth a try, though.