If you put the number in again another time (if you forget what you did or didn't cancel) it will show you the reservation and have the word cancelled next to it.
You can still log in with your account and then it gives you the option to enter your dining reservation number and the phone number it was made under. It will retrieve it then. I made some without a resort reservation number tied to them and I was still able to pull them up signing in to my account and my resort ressie.