This email is in regard to the event _________ (mention event details).
Courteously, I would like to bring to your attention that I was invited as a __________ (guest/guest speaker/any other) at __________ (name of the event). I deeply apologize for not showing up at the event and I regret the inconvenience caused to you and the management team during the event. The reason for my absence was __________ (reason for not attending the event).
Once again, I apologize to you for this unprofessional behavior and I hope that everything went well. I would love to be a part of this event in the future.
Thank you for your time and understanding.
___________ (Contact details)