Naruto could initially be angry. He might think Sakura overreacted. But then, after a moment, he would probably realize his own mistake if there was one. He'd then try to play it off with a joke to ease the tension between them, like he always does in tough situations.
Well, it could be that Naruto misinterprets something Sakura says to Sasuke and makes a comment that offends her deeply. Sakura, being the strong - willed character she is, doesn't hold back and punches Naruto. This could then start a chain of events where Naruto tries to make amends, and through this process, their friendship becomes stronger as they understand each other better.
In a fanfic, if Naruto punched Sakura, it could be due to a major misunderstanding. Maybe Sakura was being controlled by an enemy's jutsu and Naruto, in his haste to stop her from doing something dangerous, resorted to physical force. This would create a huge rift in their relationship, and Sasuke might get involved to either side with Naruto or defend Sakura.
The betrayal could lead to Naruto having trust issues not only with Sakura but also with others in the future. He might become more guarded and less likely to open up to people. For Sakura, if she comes to her senses later, she would have to face the enormity of what she has done. She would have to grovel for Naruto's forgiveness, but it would be a long and difficult road to regain his trust.
Another plot could involve jealousy. If Sakura becomes jealous of Naruto's growing popularity and power among the villagers, she might betray him. She could spread false rumors about him, which would lead to the villagers turning against Naruto.
He could become more isolated. After being punched, his friends might avoid him for a while. For example, if Harry punches him, Harry might need some time to cool off and not want to be around Ron. This would make Ron feel really lonely and regret his actions that led to the betrayal.
One possible plotline could be that Clint is on a mission. He gets ambushed by a group of enemies. First, he's punched during the initial struggle. Then, as he tries to fight back, one of the enemies stabs him with a knife. Just when he thinks he might escape, he gets shot by a sniper. His fellow teammates then have to race against time to save him.
Clint would probably be in a lot of pain, but his training would kick in. He might first try to assess the severity of his injuries. If the punch didn't daze him too much, he'd immediately start looking for a way to counter - attack those who are attacking him. Even if he's been stabbed and shot, he'd use his skills to find cover and plan his next move.
It could be Draco Malfoy. In some fanfictions, Draco might be pushed to such an extreme by his family's beliefs that he resorts to physical violence against Hermione. He might punch her out of jealousy or to assert his so - called superiority.
If Naruto gets hurt by Sakura, their relationship could become strained. Naruto might be confused and hurt, both physically and emotionally. He might start to avoid Sakura at first.