I work with pediatrics and conduct / attend clinics in different hospitals and other facilities every day of the week.
When a appointment is made, a time slot, space, and the time of various healthcare professionals is set aside just for you.
You are asked to give at least 24 hr notification if you can't make it.
When there is a "No Show" , as you did, then there is wasted time (and money) and someone else who could have used your slot.
Imagine a hair dresser in a salon who depends upon her appointments showing up.
All I'm saying is be happy they got you an appointment when they did. You are probably at the bottom of their list. Would not expect any special treatment right now.