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