It is a little marginal. It would have been simpler if you did n

contact them. If you want to keep their money, what you should do is to do nothing. If they contact you within a reasonable time (say 24 hr) saying they have some decent excuses and they want to remake the appointment, then you have little ground to refuse them. If they did not contact you, you then are well justified to keep the money. Now you contacted them and they had a decent reason...