Dear Mrs.Zhang
　　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