One possible reason could be a misunderstanding. Maybe Sakura said something that Naruto misinterpreted as being hurtful or dismissive of his feelings. For example, if she made a comment about his abilities in a negative way during a crucial moment, it could lead to him distancing himself.
Another reason might be that Naruto realizes his feelings for Sakura are unrequited and it's too painful for him to be around her. He may decide that cutting off communication is the best way to deal with his heartache. He could be thinking that by not talking to her, he can move on and focus on other aspects of his life, like his ninja training or his relationships with other friends.
One possible reason could be that Naruto's character development leads him to focus more on other aspects like his ninja duties or his bond with other friends. As he grows, his priorities shift.
One possible reason could be that Naruto has faced so many betrayals and hardships in the fanfiction. For example, if his friends turned on him for some contrived reasons, it might lead him to stop caring. Another reason could be the constant failure in achieving his goals, like protecting the village but always being undermined by political intrigue within the fan - made story.
Sakura might be initially confused. She could be thinking about what she did wrong to make Naruto stop talking to her. Since they have been friends for a long time, this sudden change would be very puzzling for her.
One possible reason could be that Naruto has finally realized that Sakura's heart has always been with Sasuke. After years of chasing after her and getting constantly rejected or overlooked, he just gets tired and decides to move on. He might start to focus more on his own personal growth and the other relationships in his life that are more reciprocal.
One possible reason could be a misunderstanding. Maybe Hermione does something that Harry misinterprets, like siding with someone he doesn't trust in a Hogwarts issue. Another reason might be jealousy. If Hermione starts getting closer to another character, Harry could feel left out and stop talking to her. Or perhaps a dark influence affects Harry's mind, making him think Hermione is a threat to him in some way.
One possible reason could be that the author wants to create a sense of mystery. By making Harry Potter stop talking, it makes the readers wonder what he is thinking or feeling, and adds an air of enigma to his character. Maybe he has witnessed something so shocking that he is rendered speechless for a while.
One reason could be Sasuke's continued pursuit of revenge. If in the fanfiction, Sasuke is so consumed by his need for revenge that he neglects Sakura and their relationship, it might cause her to stop loving him. Another reason could be that Sakura discovers a side of Sasuke she doesn't like, like a cruel streak.
One possible reason could be that Naruto finally realizes Sakura's true feelings for Sasuke are unwavering. He might come to understand that his constant pursuit is fruitless and decides to respect her feelings and stop asking her out.
One possible reason could be that Sakura realizes she has different goals or values that are not compatible with being around Naruto. Maybe she discovers a new path in life that doesn't involve him, such as focusing on her own medical ninja career without Naruto's chaotic lifestyle interfering.
One possible reason could be that the fanfiction author wants to focus on other characters. Maybe they feel they've explored Naruto's character enough and want to give more spotlight to characters like Sasuke or Sakura. Another reason might be that the plot of the fanfiction takes a turn where Naruto's acting skills are no longer relevant or needed.