She would be devastated. Hermione is a very emotional person deep down. She might isolate herself at first, not wanting to talk to anyone, even her closest friends like Harry and Ron. She'll probably go through a phase of denial, not believing that her parents are really gone.
She might also become more protective of her friends. After losing her parents, she realizes how precious life is. So she'll be on high alert for any threats to Harry and Ron. She could also start to question the safety of the Muggle - borns in the wizarding world and become an advocate for better protection for them.
Often, Hermione reacts with a mix of shock and anger. She's a strong - willed character, so she doesn't just take it lying down. In some fanfictions, she immediately confronts the cheater and gives them a piece of her mind.
Hermione would probably be defiant at first. She's not one to take injustice lying down.
Hermione could become withdrawn. She might isolate herself from her friends for a while, spending her time in the library trying to make sense of it all. She might also feel a sense of betrayal, especially if they had some sort of understanding or relationship developing.
She might initially be shocked and scared. But then she would start to think rationally and try to find a way to outwit Snape. For example, she could start to gather evidence against Snape's own wrongdoings, if there are any, so that she can turn the tables on him.
It could be that Voldemort's followers target Hermione's parents to get to her. They use a dark spell that slowly drains their life force. Hermione discovers this too late and has to face the guilt along with her sorrow. In the fanfiction, she could then seek revenge or focus on using her knowledge to prevent other wizarding families from suffering the same fate.
They could have been put under some sort of spell by the Order to forget about Hermione. In that case, they would have just gone on with their lives, completely unaware that their daughter had been taken. However, if the spell was not perfect, they might have had some strange feelings or dreams about a missing child.
She could get angry. Hermione, being the strong - willed character she is, might feel betrayed that Harry is leaving. She might yell at him, asking him why he's doing this and how he could leave her after all they've been through together. Her anger is really a mask for her deep sadness and fear of losing him.
She might be initially confused. Hermione is a logical person, and without an obvious reason for Harry's silence, she would be at a loss. She would probably try to think back on their recent interactions to figure out what went wrong.
She might be initially shocked. After all, Draco is usually seen as this tough Slytherin. But then she could feel sympathy. For instance, she might offer him a handkerchief without saying much at first.
She often tries to ignore it at first. She's focused on her studies and doesn't want to give the bullies the satisfaction of a reaction.