Go out of your way to apologize for missing an appointment--and have a good excuse. You should call as soon as possible, then follow up with a letter. No one wants to do business with someone they can't count on, so it's imperative that you get back in their good graces.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I can't tell you how sorry I am for missing our appointment <scheduled appointment>. <Reason for missing the appointment.>
<How you plan to make it up.> <Confirmation of next scheduled meeting.>
Thank you for your understanding and patience. I look forward to seeing you soon.
encl: <List of enclosed items goes here>