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