Hello Mr. Smith:

Thank you very much for inviting me to_________（写上本来要去干什么,可以填"the party" “the show"之类的）. I sincerely apologize for not being able to show up due to ____________________________________（这句写原因，因为你的问题中没有写明原因是什么...我也不知道怎么写...如果你也不知道的话，可以用"unexpected circumstances"）I truly hope that you can forgive my absence.

Apologies,

这里写你的名字。