Dear Jehan,
I'm sorry to say that I won't be able to make it to your party. I'm really sorry for the inconvenience. I'm currently dealing with a family emergency and I need to stay home to take care of it. I wish I could be there to celebrate with you, but I'm sure you'll have a great time without me.
I hope you have a wonderful time and I'm sorry again for not being able to make it.
Take care,
(Your name)