Restart by Gordon Korman
A bad fall has left eighth-grade Chase Ambrose with amnesia. He can’t remember anything about his life before the accident. As he slowly pieces together clues about his life, Chase realizes that he was a star athlete, team captain, and a bully. He is horrified to discover that people are afraid of him. As he becomes reacquainted with his two best friends, Chase wonders what they ever had in common.
Sidelined from the football field by his concussion, Chase begins spending time making videos with one of his former targets, Brendan Espinoza. He joins the video club and finds himself facing the wrath of Shoshanna Webber, whose twin brother was so victimized by Chase and his friends that he left home to attend music school.