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.
Sakura might become determined to win Naruto's attention back. She could start to work on herself, improve her skills or knowledge, hoping that Naruto will notice her efforts and stop ignoring her. Maybe she'll also try to help Naruto in a more subtle way without him realizing it at first.
One possible reason could be that something happened during the Kage Summit that made Naruto lose trust in Sakura. Maybe she made a decision or took an action that he didn't agree with. For example, if she sided with someone against Naruto's ideas regarding the situation at the Summit.
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.
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.
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.
Sakura might initially be confused. She has known Naruto for a long time and his sudden coldness would be unexpected. She may try to reach out to him to find out what's wrong.
She might be heartbroken. Sakura could initially be in shock and then fall into a deep sadness. She may blame herself for Naruto's leaving, thinking that she could have done something differently.
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 might be initially shocked. Sakura has always seen Naruto as a friend, so learning of his romantic feelings could completely change her perspective of him. It could make her re - evaluate all their past interactions.
Sakura might be in shock at first. Unable to believe that Naruto, who has always been so strong - willed, could have ended his own life. As she processes it, she could become angry, not at Naruto but at herself and the situation that led to this tragic outcome.
She might be really hurt and start to question her self - worth. Maybe she becomes more withdrawn and less confident in herself.