It is common courtesy to cancel an ADR if you can. But sometimes you can't do that. Things can happen at the last minute. As long as this was not an ADR that required a credit card in advance, its not a big deal. Remember, an ADR means that you will be given the next available table for your party size. They are not holding a table, waiting for you to arrive.
Last May I called to cancel an ADR at Captain's Grille about 24 hours in advance. (I had 'double booked' two days in a row, then cancelled the one I didn't need.) Anyway when I called , the CM was polite and did what I asked but I had the feeling that he thought I was crazy for bothering to do it.
things happen. If you know your plans are changing and you can call, that is great, someone else may be able to grab that last minute ressie (if it is an ADR that requires a CC, and you are a no show, they will charge you accordingly)
I have to confess, I slept through my breakfast ressie the morning after MNSSHP