Snape could be initially shocked. He's used to seeing Harry as a rather tough and rebellious student. So, seeing Harry cry might take him aback for a moment. Then he might become a bit more guarded and try to figure out if Harry is faking it or not.
Maybe Harry is under a great deal of stress. He has so many burdens on his young shoulders, like fighting Voldemort, protecting his friends, and dealing with the secrets of his past. Snape has always been a rather intimidating figure to him, but in that moment, the pressure might just be too much, causing him to break down in front of Snape.
One reason could be the stress of fighting Voldemort. Harry has so much on his shoulders, and McGonagall is a figure of authority and comfort. He might break down in front of her when the pressure becomes too much. For example, when he has to face difficult decisions like whether to sacrifice himself or when he feels responsible for the safety of his friends.
Snape might be initially taken aback. He could hide his surprise behind his usual stern facade and perhaps try to maintain order, but secretly be a bit worried.
In some fanfictions, Snape might awkwardly try to comfort Harry. Given his own complex personality, it wouldn't be an easy or natural gesture for him. He might pat Harry's shoulder stiffly or offer some gruff words of reassurance like 'Pull yourself together, Potter.'
One possible reason could be a sudden outburst of pent - up emotions. Maybe Hermione has been bottling up her feelings for a long time, and in the presence of Draco (in the context of the fanfic), something triggers her to break down. It could be a situation related to the ongoing conflict between Gryffindor and Slytherin in the Hogwarts setting, and Draco represents all the stress and frustration she has with that divide.
She might immediately comfort him. McGonagall is a strict but kind teacher. She could put a hand on his shoulder and say soothing words like 'There, there, Mr. Potter. It will be all right.'
It could improve their relationship significantly. This moment of vulnerability from Harry and acceptance from Snape could break down some of the long - held animosity between them. They might start to see each other in a new light and become more understanding of each other's actions in the future.
Draco might be initially shocked. He's used to seeing Hermione as a strong adversary, so seeing her cry would be unexpected. He could just stand there, not knowing how to react at first.
Sirius might immediately pull Harry into a tight hug. He would be very protective and try to comfort him. He might say some soothing words like 'It's all right, Harry. I'm here for you.'