Subject: Sorry for Missing Your Birthday Celebration
Hi [Friend's Name],
I hope you're having an amazing birthday! I wanted to let you know how truly sorry I am for not being able to make it to your party. I was really looking forward to celebrating with you, but unfortunately, I can't come because [reason, e.g., I'm not feeling well, or I have a family commitment that I can't reschedule].
I hope you have a fantastic day filled with laughter, joy, and everything you love. You deserve all the happiness in the world. Let's catch up soon so I can hear all about the party and celebrate together in our own way.
Wishing you all the best,
[Your Name]