Simply replace the unfilled areas such as "" and "[date]" with your information.
Please accept my apologies for missing the [type of event] that you held on [date]. I hope my last-minute change of plans did not inconvenience you too much.
As you know, I had planned on attending and was looking forward to it. However due to unforeseen circumstances I could not attend.
Once again, I ask for your understanding in this matter and hope that my last-minute call to bow out was acceptable.