Dear Sir or Madam,
This is ----(你的名字). The purpose for me to write this letter is to sincerely express my apology. I feel really regret that I will not be able to (不能去干嘛). Here are my explanations as followed.
Due to (原因)，I could not 不能去干嘛 any more. For my part, please let me know what I could do anything to make it up and I will try my best to assist with it. I hope that the settlement of this letter will meet your wishes. Hope you can forgive me for my negligence.
Once again, I am really sorry for any inconvenience that I caused. Please accept my sincere apologies, and if that possible we could arrange a new schedule. I am looking forward to hear your favorable reply.