The teachers, like Professor McGonagall, would be concerned but also take charge. She might use her magic to assess Hermione's condition and then arrange for her to be taken to the hospital wing. Other students would be a bit in shock. Some of Hermione's female friends might start crying, while the boys would look on in concern, not really knowing what to do.
Harry would probably be very confused at first. He knows Hermione is usually so studious and responsible. He'd likely try to talk to her to find out why she did it. Ron, on the other hand, might be a bit more blustery. He'd probably say something like 'Hermione, what were you thinking?' and be quite shocked. The professors would be disappointed and would likely call her in for a stern talking - to, as Hermione has always been a model student.
In fanfiction, one reason could be over - studying for her classes at Hogwarts. Hermione is known for her studious nature, and she might push herself too hard, staying up all night to master difficult spells or research complex magical theories. Another reason could be her involvement in a dangerous adventure. If she's constantly on the run from dark wizards or trying to solve a mystery, the stress and lack of proper rest could cause her to pass out from exhaustion.
The Twilight characters would be quite shocked. Vampires like Edward might be very suspicious of Hermione's new status as an imprinted character. Werewolves like Jacob would have a different reaction. Since imprinting is a big part of their culture, they might be more accepting but also cautious as Hermione is an outsider from a different fictional universe. They would need to figure out how to incorporate her into their pack or deal with her in some way.
Well, the other students in Gryffindor would likely be a bit taken aback at first. But then, knowing Hermione's intelligence and her tendency to be different, they would probably accept it. They might be curious about how she's going to manage to keep up the charade. The Hufflepuffs would be generally kind and understanding, perhaps even offering to help her if she needs it. The Ravenclaws would be intrigued by the idea and might study her transformation from a more academic perspective. The Slytherins, as a whole, would be more likely to try and use her disguise against her, either by blackmailing her or trying to expose her in a way that would cause her and her friends embarrassment.
Harry would be crushed. He'd feel like he lost a sister. Ron would be in total despair, probably blaming himself. Ginny would be heartbroken too as Hermione was like a sister to her. The Weasley family, who had accepted Hermione as one of their own, would be in mourning. They'd all wonder how they could have prevented it and would be angry at the unfairness of it all.
Harry would be extremely worried. He'd probably rush to her side immediately and try to do whatever he can to help. He might blame himself for not being there to protect her. Ron, on the other hand, would be in a state of shock at first and then would try to use his humor to lighten the situation a bit while still being very concerned. Hagrid would be devastated. He sees Hermione as one of his students he cares about, and he'd likely bring some sort of magical creature - related remedy, thinking it could help.
Hermione would probably be defiant at first. She's not one to take injustice lying down.
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.
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.
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.