RESTART by Gordon Korman

Chase's memory just went out the window. Chase doesn't remember falling off the roof. He doesn't remember hitting his head. He doesn't, in fact, remember anything. He wakes up in a hospital room and suddenly has to learn his whole life all over again, starting with his own name. When he gets back to school, he sees that different kids have very different reactions to his return. Some kids treat him like a hero. Some kids are clearly afraid of him. Pretty soon, it's not only a question of who Chase is; it's a question of who he was, and who he's going to be.