What happened to Melinda that caused everything to change?
In Speak, Melinda was raped at a party over the summer; however, she hasn't had the strength to tell anyone. After it happened, she called the police, not realizing the devastating consequences making that call would have.
Because the police responded to the 911 call, the party was broken up, and many people got in trouble. Since Melinda has never explained to anyone what happened to make her call 911, the party goers are angry with her for bringing the police's attention and presence, and they hold it against her.
As a result, on the first day back to school after summer vacation, everything is different for Melinda. Her "best friends" won't even speak to her, and they are all undergoing changes of their own. She navigates much of the school year on her own, learning things about herself and realizing how important it is to speak up.