You most likely cannot: while the dentist did not have the crowns ready on the specified date, he did have them later. Under those circumstances, the failure to have them on the specified date is probably not a material (critical) breach--the sort of breach which justifies the other side to treat a contract as terminated--or alternately, was a material breach but was cured (corrected) by then getting the crowns in later. Either way, you probably cannot treat the contracy as terminated by the dentist's breach and get a refund, but rather would have to reschedule the appointment and give your dentist the chance to put in the crowns. If the dentist had never gotten them in, that would be different, as it would be if the failure to have them by the specified date caused you some significant physical or economic injury--but that does not appear to be the case.