Well, if Umbridge uses Veritaserum on Harry in a fanfic, it depends on how the author writes it. Some fanfic writers might have Harry spill out information about the Order of the Phoenix, which could put the whole organization at risk. Or perhaps he'd talk about his experiences with Voldemort that he's been keeping to himself. This could potentially change the whole dynamic between the characters in the fanfic, especially if Umbridge uses this information to her advantage.
After Umbridge uses Veritaserum on Harry in a fanfic, the plot could develop in various ways. If Harry reveals secrets about the resistance against Umbridge, she could crush it. On the other hand, his friends might try to rescue him from the situation and that could start a whole new adventure of escaping Umbridge's clutches.
One major consequence could be that Harry reveals his connection to Sirius Black. If Umbridge gets to know about this through the Veritaserum, she could use it as a weapon against Harry. She might try to capture Sirius or use his existence as a way to blackmail Harry into following her every order. Also, Harry could end up revealing the details of how he has been learning magic outside of the school curriculum, which would be a huge violation in Umbridge's eyes.
Dumbledore, if he finds out, would surely take action. He might not be able to reverse the effects of the Veritaserum immediately, but he would find a way to protect Harry from any long - term consequences. The other members of Dumbledore's Army would be in shock. They would worry about what Harry has revealed and also feel a sense of guilt for not being able to protect him from Umbridge's devious plan. Some of the more rebellious students might even start a protest against Umbridge's use of Veritaserum on a fellow student without proper authorization.
Harry might initially be shocked and try to resist. But since Veritaserum is powerful, he would soon start answering truthfully. He could look angry towards Snape for using such a potion on him without his consent.
One possible consequence could be that Harry reveals some deep - hidden secrets. For example, his true feelings about his friends or his fears regarding Voldemort. Since Veritaserum forces the drinker to tell the truth, Harry might disclose information that he has been keeping to himself, which could then change the way Snape views him or the actions he takes in relation to Harry.
One possible consequence could be that she gets fired from her position. If Harry manages to expose her actions, the wizarding school or the Ministry of Magic might not tolerate such blatant abuse of power and take disciplinary action against her.
Well, in a fanfic, if Harry is given Veritaserum, he might reveal all kinds of secrets. Maybe he'd spill the beans about his true feelings for his friends, like how much he really trusts Hermione and Ron. He could also disclose some hidden details about his battles with Voldemort, things that he's kept to himself because of fear or just not having the right moment to share.
In the context of the original Harry Potter story, it seems quite abnormal as Umbridge is a very unlikable character to Harry. But in the world of fanfic, anything can happen. Fanfic allows for different interpretations and character developments, so while it's not normal in the traditional sense, it's possible within the creative space of fan - created stories.
Another way Harry could respond is with a sense of shock and disbelief at first. Given that Umbridge is supposed to be in a position of authority, Harry might be momentarily stunned that she would go to such lengths. However, his Gryffindor courage would soon kick in, and he would likely start to resist, perhaps by using his magic in creative ways to protect himself or to disarm Umbridge, all while trying to uphold the values of the wizarding world that oppose such cruel and unjust treatment.