【正确答案】Dear Prof. Li,
First of all, please allow me to express my deep sorry 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 useless. However I do not want you to misunderstand me. On my way to your office, my roommate called me that he broke his leg and needed help. I stopped a taxi and went back my dormitory to help him, then sent him to a nearby hospital. I stayed there until his brother 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 time to meet me
I am looking forward to hearing from you