I hate to do this at such late notice, but I'm afraid that I have to cancel my plans to attend {meeting/trip/event}. I am so sorry for any inconvenience this causes you and any changes that it necessitates in your current plans.
I would never do something like this so last minute, but unfortunately {reason for cancellation}. I hope you understand that I never saw this emergency coming and was completely unprepared for it.
Thank you for your understanding in the matter and I hope you have a wonderful time at {event}.
Sincerely,