Perhaps he has been through a very tough battle, physically and mentally exhausted. He could be suffering from some sort of post - battle trauma. In this state, he would be more likely to be quiet as he tries to recover and come to terms with what he has experienced.
One possible reason could be overusing his special powers like the Rasengan or Sage Mode. These techniques might have some sort of negative impact on his eyes over time. For example, if he doesn't control the chakra flow properly during these powerful moves, it could damage his optic nerves gradually.
One possible reason could be a powerful new enemy. Maybe a villain with an unknown and extremely deadly jutsu appears. This enemy could be so strong that Naruto, despite all his power, is no match for them. For example, an enemy who can nullify Naruto's chakra or has the ability to manipulate time and space in ways that Naruto can't counter.
It could be due to some sort of trauma. Maybe he witnessed something really bad that made him retreat into himself and become quiet. For example, if he saw someone he cared about get hurt badly, it could have silenced him as a form of self - protection.
One reason could be that he's going through some internal struggles. Maybe he has secrets he doesn't want to share. For example, he might be worried about burdening his friends with his own problems.
In the fanfic, it might be that Naruto has new responsibilities in the village that are more important than chasing Sasuke. For example, he could be needed to deal with new threats to Konoha, so he has to let go of his personal mission to bring Sasuke back.
It could be due to the failure of protecting his village or his loved ones. Naruto has always been about protecting those important to him. If he fails at that, his anger is understandable.
One reason could be to show a different side of Naruto. The original Naruto is very social and friendly, but in a fanfic, making him antisocial can create a contrast. Maybe the writer wants to explore how his friends would react if he suddenly changed.
His friends would be worried. Sakura, for instance, might try to reach out to him and get him to open up. She could be constantly checking on him, bringing him food or just sitting with him in silence, hoping he'll start talking again.
One reason could be the fear of losing his friends. Naruto has always been very protective of his comrades. In some fanfictions, he might face a situation where he is on the verge of losing them, like a powerful enemy threatening to kill them all. This would make him afraid as his friends are his most precious things.
Typically, Naruto quiet fanfiction often focuses on the more subdued aspects of the Naruto world. It might center around characters who are introverted or who have a quieter presence in the original story. For example, it could be about a character who is more of an observer than a fighter.