On Naruto's side, he would probably feel guilty after realizing what he had done. This guilt could make him try to make amends, but Sakura's initial reaction of anger and hurt would still be a big obstacle in their relationship. Their interactions would be filled with tension and awkwardness.
Perhaps there was some sort of dark influence at play. In the world of Naruto, there are often dark forces and genjutsu (illusion techniques). It's possible that Naruto was under the influence of a genjutsu that made him see Sakura as a threat, so he punched her in self - defense without realizing it was an illusion.
It could become more strained at first. Sakura would be hurt, both physically and emotionally, and she might hold a grudge against Naruto. However, if Naruto had a good reason for punching her, like protecting her from a greater danger she didn't see, she might eventually come to understand and their relationship could become stronger in the long run.
In the fanfic, Sakura might be influenced by some dark force or a bad character. She starts to act against Naruto and their friends. Naruto, being the hero, tries to reason with her first but when she attacks him, he punches her back. This punch snaps her out of the influence, and they join forces again to fight the real enemy.
In some fanfictions, Sakura's punch might be part of a comical situation. For example, Naruto says something really stupid and Sakura just can't help but punch him. But this punch then leads to a series of adventures as Naruto tries to get back at her in a friendly way.
I recommend 'The Punch that Changed Everything'. In this fanfiction, Sakura's punch makes Naruto see the error of his ways regarding a certain mission. It's a short but impactful read.
Well, one possible consequence could be that Naruto is seriously injured. Maybe he has a broken bone or gets knocked out cold. It could also lead to some sort of rift between them, with Naruto feeling betrayed by Sakura's overly aggressive action.
Well, Naruto would probably be in a great deal of pain. He might double over and be unable to move for a while. It could also lead to a big argument between him and Sakura, with Naruto feeling betrayed and Sakura perhaps realizing she overreacted.
There could be many unknown authors writing such a fanfiction. It might be a new and emerging writer in the Naruto fanfiction community who just had this interesting and somewhat out - of - the - box idea. It's really hard to determine a specific author without more information.