Airhead by Meg Cabot. (again)
I think I've read this book five times already. Everything started when a tomboy named Em Watts was forced to go to the new SoHo Megastore grand opening with her little sister Frida. Everything in Em's life was great (except for getting her best friend, Christopher Maloney to notice she's actually a girl and not that sexless friend) until the accident occurred. One bizarre accident later, and Em Watts, always the tomboy, never the party princess, is no longer herself. Literally, because her body was transplanted into the body of the Face of Stark, Nikki Howard. The book is really fun. Em has this whole mystery to solve about the real reason why the real Nikki Howard "died" and this whole dilemma about Christopher to notice her and of course, how to accept the truth that she is no longer the identity she used to be and to accept the truth that she is now Nikki Howard.