Apology for Missing Event

Dear {Recipient},

Please accept my apologies for not showing up at {event} {yesterday/last weekend/etc.} despite RSVPing “yes.” I was really looking forward to the event; sadly, I wasn't able to attend at the last minute due to {reason for absence}.

I'm really sorry I couldn't be there to celebrate {occasion} with you. I didn't {call/text} you anymore because I didn't want to bother you on your big day. Anyway, I wish you {a happy birthday/all the best/etc.} and I hope we can catch up soon!

Warmly,
{Your Name}