then kisses her twice, hard. Because she's drunk, she doesn't quite realize what's happening, but then they're on the ground, and he rapes her; as she struggles and tries to scream, he covers her mouth. Once it's over, she calls the police for help, sealing her fate as a social outcast. Because everyone judges her and bullies her for calling the police, she doesn't feel like she can speak out about what happened to her.

From page 135 of Speak:

We were on the ground. When did that happen? "No." No I did not like this. I was on the ground and he was on top of me. My lips mumble something about leaving, about a friend who needs me, about my parents worrying. I can hear myself—I'm mumbling like a deranged drunk. His lips lock on mine and I can't say anything. I twist my head away. He is so heavy. There is a boulder on me. I open my mouth to breathe, to scream, and his hand covers it. In my head, my voice is as clear as a bell: 'NO I DON'T WANT TO!' But I can't spit it out. I'm trying to remember how we got on the ground and where the moon went and wham! shirt up, shorts down, and the ground smells wet and dark and NO!—I'm not really here, I'm definitely back at Rachel's, crimping my hair and gluing on fake nails, and he smells like beer and mean and he hurts me hurts me hurts me and gets up

and zips his jeans

and smiles.

The major incident that happened to Melinda was that she was raped.  

That particular incident happens before the events of the book begin.  In the summer before beginning her freshman year of high school, Melinda went to a party for high school students attending Merryweather High School.  The party had students of all ages there, and alcohol was present as well.  Melinda had been drinking too, but she did not give sexual consent to Andy Evans.  Andy Evans raped Melinda.  Melinda called the police from the party, and consequently all of the students at Merryweather High School treat her very poorly.  Over the course of the novel, Andy antagonizes Melinda to no end and tries to rape her a second time.  His actions are discovered, and people now realize what actually happened to Melinda at the party. 

In the novel Speak, by Laurie Halse Anderson, the protagonist Melinda Sordino narrates her first year of high school. After calling the cops on a party over the summer, Melinda has been abandoned by her former friends and is unpopular with her peers. It is also clear that Melinda is struggling with some sort of secret trauma.

Eventually, it is revealed that Melinda's depression and anxiety stem from a sexual assault. Over the summer, Melinda attended a party where older high school students were consuming alcohol and dancing. Melinda gave into peer pressure and drank several beers before an attractive, popular senior named Andy Evans approached her. Andy began groping Melinda and forcefully kissed her, which made Melinda feel uncomfortable and confused. He then raped Melinda, leaving her traumatized, afraid, and disoriented.

Afterward, Melinda reacted by instinctively calling 911, though she was too upset to tell the operator what happened. The police then arrived and broke up the party. The teenagers at the party learned that Melinda called the police but do not know why, leading her to become a social pariah at school.

Isolated and unable to cope with the trauma from her assault, Melinda cannot bring herself to verbally express what happened to her friends and family. Eventually, she finds it difficult to speak altogether. Melinda internalizes her pain and becomes an outcast, losing her former friends, allowing her grades to slip, and becoming increasingly silent.

As time passes, Melinda begins to emotionally heal, using art as an outlet, and she eventually gets to a point where she can speak about the traumatic incident.

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.