Dear___________,
    I am terribly sorry to tell you that_________,and I am writing this letter to show my deep regret.Please accept my sincere apology with gratitude.I hope you will understand my situation and excuse me for___________.
    The reason why__________is that__________.Under that circumstance,____________.Therefore,it was not my power to___________.
    If possible,I would like to suggest that___________.I shall be obliged if you will kindly write and tell me___________.I am looking forward to seeing you again.
    Yours,
    ________