This may have just been an exercise in writing English, but I'll answer the question in case you were also asking a question about social norms. Call and apologize as soon as possible. In the US, I know that some doctors will refuse to see you again if you miss an appointment without calling first, but not all of them. I think it also depends on whether you have a good relationship with the doctor already. If you do have a relationship, he is more likely to overlook an unusual and unintentional mistake.