I found out about my surprise party for my birthday, what should I do?
You go to your party and act like you know nothing about it! The people who planned your party have gone to a lot of trouble to provide you with a lovely gift. I think they would be upset if they knew it wasn't a surprise. I have had the same situation happen to me many years ago. I acted surprised and enjoyed myself very much.
Then, write kind thank you notes to the people who took the time to plan such a sweet party. I'm not talking emails, either! A hand written note is appropriate for this situation. You can take that much time for a thank you note to people who cared enough to organize your party.
Having said that, I am an old mom who thinks old-fashioned ways are still good ways! They are also appreciated.