Did you cancel the flight before flight time of first flight, either on the website, or by calling.

If so, you should be able to log into your mileage plus account (if you have one) and go to look at your itineraries, then click on the "past" tab and your old reservation number should be there, and allowing you to rebook. Full value will be shown, but at time of rebooking, you will need to pay the change fee - usually $150 for a domestic ticket.

If you didn't cancel first, I'm afraid the value is going to be gone. If you don't cancel by flight time, you lose the value of the ticket.