Dear {Recipient},
I am so sorry that I was unable to attend {name of event}.
You might laugh when you hear the reason I was unable to go. {Story about not attending} Crazy, right? I know, I know. That could only happen to me.
I promise, at a future {name of event}, I will be there with bells on, ready to make up for all I missed this time around. So don't hold this against me, okay? I'll be expecting my invitation when you do this again.
Until next time,
{Sender}