Lizzie McGuire Mysteries, Case of the Missing She-Geek, Book 3 by Lisa Banim

In a (failed) effort to be cool in school, Lizzie hurts the feelings of major she-geek Audrey Albright. It's a total mistake and Lizzie wants to apologize-but Audrey's disappeared. She was last seen in the girls' room, crying. Then she doesn't come to school for days. With Gordo and Miranda in tow, Lizzie follows a trail of clues to a science-fiction convention, where Larry Tudgeman becomes their guide through the underworld of geekdom. But is Lizzie really ready to go where no cool kid (like ever!) has gone before?