At the very start, Snape would likely sneer at Hermione's request. However, as he realizes the seriousness of the matter (say, if it has to do with a threat to the school or a powerful dark magic item), he might change his tune. He could be grudgingly impressed by Hermione's courage in coming to him, a person she doesn't particularly like and who doesn't like her. Snape's reaction would be complex, a mix of his usual disdain for Gryffindors, his own hidden motives, and the potential importance of Hermione's request.
Snape would probably be quite surprised at first. Hermione is known for her academic prowess and self - sufficiency. His reaction could range from being dismissive, thinking she should be able to solve her own problems, to being intrigued by the nature of her request. If it's something related to the dark arts or a mystery in the wizarding world, his interest might be piqued, but he'd still be guarded in his response. He might even try to use her request as a way to gain some advantage or information for himself.
He could also be a bit sarcastic in his response. Snape has a sharp tongue, so he might say something like 'Miss Granger, what makes you think I have the time or inclination to help you?' But as Hermione explains her situation, he might gradually soften, realizing the seriousness of the matter. Maybe he starts to see a bit of himself in Hermione's thirst for knowledge.
Snape would probably be very cold and indifferent at first. He might even think it's some sort of Gryffindor ploy to get his attention or make him look bad. He has always had a low opinion of Hermione's house, so his initial reaction would be guarded.
He might be initially suspicious. Snape is a complex character. He's used to being on his guard, especially with Gryffindors like Hermione. So, his first reaction could be to wonder what Hermione's real motives are for seeking his help.
Snape might initially be shocked. He's not one to show much emotion openly, but internally he could be very disturbed. He might start investigating what happened to Hermione immediately, using his skills in potions and magic to try and find a cure or at least understand the cause.
She might decline. Hermione is a very practical and goal - oriented person. She has her studies and her friends to think about. Going on a date with Snape, who has a rather complex and not - always - friendly reputation, might seem too risky and not in line with her plans. Also, she may not have any romantic feelings for him at all.
He might initially show annoyance. After all, Hermione is often seen as a know - it - all in his eyes. But his sense of justice and his underlying good nature (despite his cold exterior) would kick in. He'd untie her in a rather brusque manner, but then start to question her about how she got into this mess. Depending on her answer, he might offer some cryptic advice on how to avoid similar situations in the future.
One possibility is that Snape initially refuses to help Hermione. He has his own grudges and is often portrayed as a cold character. But as the story progresses, he might see something in Hermione's situation that relates to his own past or values, and reluctantly start to offer some guidance.
Snape might initially be taken aback. He's used to students being intimidated by him, so Hermione's outburst could shock him into momentary silence. Then he might retaliate with his usual sarcasm, trying to put her in her place.
Snape might be suspicious at first. He might think it's some sort of trick or part of a bigger plot. However, as he observes Hermione more closely and realizes the genuineness of her situation, his attitude could gradually change to one of reluctant assistance. He might even offer some words of wisdom, albeit in his typical sarcastic way, to help Hermione regain her strength.
Snape could be very sarcastic and cold at first. He might taunt Harry about finally coming to him for help. However, as he realizes the gravity of the situation, he could start to take it more seriously and actually use his skills and knowledge to help Harry. For example, he might say something like 'Potter, so you've finally realized that you're in over your head and need my help? Well, don't expect me to hold your hand.' But then he would still start to investigate the problem.