I would suggest calling the office and explaining to them your situation. They should be able to reschedule another appointment for you at that time. If you have to leave a message on their machine because they're not in the office, you should call a second time to confirm that they got your cancelation; some offices will charge you for missed appointments.
If it's after hours or a weekend, most of the Doctors have an answering service.
Call the Dr Office
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