Harry may also feel a sense of adventure. Despite the situation, he's used to facing strange and dangerous things. He might start to think about how he can use this situation to his advantage, like finding out more about his parents or the secrets of the wizarding world that Sirius might know. He could be a bit excited as well as scared. For example, he might be thinking that this could be a chance to get away from the Dursleys for good.
At first, Harry might be extremely confused. He trusts Sirius to a certain extent, but being kidnapped is still a shock. He would probably ask Sirius a lot of questions like 'Why are you doing this?' and 'Where are we going?'
At first, Harry would likely be shocked and angry. He's used to being in dangerous situations, but being kidnapped by Fleur, someone he thought was an ally, would be a big betrayal. He'd probably try to fight back immediately, using his magic if he could.
Harry could also try to reason with Voldemort. He might remind Voldemort of the prophecies and how killing him might not be in Voldemort's best interest. He could also try to appeal to any bit of humanity that might still be left in Voldemort, although that's a long shot.
One possible reason could be Sirius believing he can protect Harry better on his own. Maybe he thinks the Dursleys are too dangerous for Harry and that the wizarding world isn't doing enough. So, he decides to take matters into his own hands and kidnap Harry to keep him safe.
Harry's reaction could be a mix of fear and defiance. He's faced Voldemort multiple times, so he won't be completely helpless. He may try to understand Severus' psyche, remembering that Severus has always had a somewhat ambiguous relationship with him. He might try to appeal to Severus' better nature if he believes there is one. Harry would also be thinking about how this situation affects the overall fight against the Dark Arts and Voldemort. He might see it as a setback or an opportunity, depending on how he can turn the situation around.
Harry would likely be defiant. He has faced Voldemort (Tom Riddle) before and he's not one to cower easily. So, he'd probably start looking for ways to escape right away, using his wits and any small magical items he might have on him.
Sirius would probably be overjoyed at first. Harry is his godson and he cares deeply for him.
Sirius would probably be very worried and immediately try to help Harry. He might use some basic healing spells he knows.
Harry would probably be very defiant at first. He has faced Voldemort (Tom Riddle) before and he won't go down without a fight. He would likely try to use his magic skills to escape or at least hold Tom Riddle off until he can find a way out.
He might be initially embarrassed, especially if he receives it in a public place like the Great Hall.