If you did not have to give your credit card number you cannot and will not be charged for being a no-show. However you really should call to cancel as soon as you know you cannot make it just to be courteous to the rest of us who may want to eat there.
I just got off the phone with a CM at Dining about this issue. Our incoming flight has been pushed back by 3 hours. We had dining reservations for 5:30 at 1900 Park Fare. Our plane isn't scheduled to land until after 4. I wanted to know what would happen if we didn't make it in time for our ADR. I specifically asked about the no-show policy. I asked if I would be charged if we didn't show. The CM told me that this isn't one of the resturaunts that will charge if you don't make it.
Hope this helps you.