Being in a car accident not good enough excuse to miss your appointment

On the day of my appointment with the doctor, I was in a car accident where my car was a write off and I had to go to hospital for medical attention. They sent me a bill for not showing up, but after I sent them messages about my accident they felt it wasn't a good enough excuse for missing my appointment and threatened with legal actions. I paid this amount but truly very unhappy about the conduct of people I trusted and expected more from.