Dear Prof. Wang,
I’m 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