Sakura might initially be confused. She would probably wonder why Naruto is acting this way all of a sudden. For example, she might try to get his attention by calling his name a few times, but when he doesn't respond, she could be left feeling a bit bewildered.
She could also get angry. Given Sakura's strong personality, Naruto's ignoring her might be seen as disrespectful. She might confront him about his behavior, demanding to know why he's being so cold. She could say something like 'Naruto, what's your problem? Why are you ignoring me?' in an angry tone.
She could be confused at first. She might not understand why Naruto is suddenly ignoring her, since they were friends before. So she might try to approach him to ask what's wrong.
He might be initially confused. Sasuke is not used to Sakura ignoring him as she has always been somewhat persistent in her attention towards him. So, he may be at a loss as to why she is suddenly acting this way.
Sakura might react by being more assertive around Naruto. Instead of being shy or holding back, she could be more forward in showing her friendship or even her deeper feelings. She may also try to intimidate the fangirls in a way, like using her strength as a ninja to show that she is not one to be trifled with.
Sakura would likely be devastated. Naruto has been her friend and teammate for a long time. His death would leave a huge void in her life, and she'd probably go through a long period of mourning.
At first, Naruto would likely be extremely shocked. He has always considered Sakura a friend and teammate, so this would be a huge blow. He might question his own judgment in trusting her.
Sakura might be completely shocked at first. She might freeze for a moment, not believing what's happening. Then she could start crying tears of joy and say yes immediately.
Another reaction could be that Sakura tries to follow Sasuke. She believes that she can change his mind or that she can be of help to him. She might leave her own responsibilities behind for a while to chase after him, thinking that their bond is strong enough to overcome whatever is making Sasuke leave.
Sakura would probably be initially very angry. She has strong feelings for Sasuke, but kidnapping is a very extreme act. So she would likely fight back, using her medical ninjutsu skills and combat abilities to try and escape.
In some fanfics, Naruto may ignore Sakura because he has his own emotional traumas to deal with. He might find Sakura's presence a reminder of things he wants to forget or move past, so he chooses to keep his distance. For example, if he has faced a lot of rejection from her in the past, he could decide to just cut her out of his life.
She may feel angry at first. Sakura is a strong - willed character. She might think that Naruto has made this decision without considering her feelings. In the fanfic, she could go on a rant about how he should have talked to her before making such a big decision.