dear prof. wang,
im writing to ask you to excuse me for not being able to keep our appointment. i do know that this is very impolite and must have caused you much trouble.
i do reckon that at present any explanation is pale and futile. however i do not want you to misunderstand me. on my way to your office, an old lady suddenly fainted due to heart attack on the bus. i stopped a taxi and sent her into a nearby hospital. i stayed there until her son came, which spoiled our appointment.
i am aware that our appointment is of importance. i do hope that you would be kind enough to spare your valuable to meet me.
i am looking forward to hearing from you.
sincerely yours