Sakura would probably be really concerned. She has known Naruto for a long time and would want to make sure he's okay. She might try to talk to him and get him to open up about his feelings.
In some fanfictions, Naruto could break Hinata's heart by choosing another girl over her. For example, if he decides to be with Sakura instead. Hinata has always had deep feelings for Naruto, and seeing him with someone else would shatter her heart. It could also be that Naruto ignores Hinata's sacrifices for him and takes her for granted, which slowly breaks her heart as she realizes he doesn't value her as much as she thought.
It could be for the sake of creating an interesting and unique plot. Writers like to take characters out of their normal situations to explore different aspects of their personalities and relationships.
One popular fanfiction might be on platforms like FanFiction.net or Archive of Our Own. You can search using the keywords 'hinata tied up naruto' on these sites. However, make sure to follow the rules and regulations of the platforms.
Naruto could be initially in shock. Given his strong feelings for Hinata, he might not believe it at first. He could try to talk to her and ask for reasons, hoping to fix things.
One possible reason could be miscommunication. Maybe Hinata felt that Naruto was always so focused on his ninja duties and didn't have enough time for her. Another reason might be external pressure. For example, if there were some political or family issues in the Hidden Leaf Village that made their relationship difficult to maintain. It could also be that Hinata's own insecurities got the best of her. She might have felt that she wasn't good enough for Naruto, who was a hero in the village.
In a 'naruto and hinata grown up fanfiction', perhaps they go on adventures outside of the village. Naruto has always been curious about the world, and now with Hinata by his side, they explore different lands. Along the way, they meet new friends and foes, and Hinata's gentle yet strong nature helps them out of many difficult situations.
In such a fanfiction, perhaps the plot could revolve around Naruto and Hinata starting a family. It might show their journey through Hinata's pregnancy, with Naruto being overly protective and excited about becoming a father. They could face challenges together, like dealing with the reactions of their friends and family in the ninja village.