He could regain it through the support of his friends. If Hermione and Ron show him unconditional love and support, and remind him of all the good he has done and all the people he has saved. Their friendship has always been a strong bond in his life, and it can be the key to bringing him back from the brink. Also, if new friendships are formed, like with some previously overlooked characters who offer him new perspectives on life.
Perhaps he is betrayed by those he trusts. In the original series, there were instances of betrayal, and if in the fanfiction this is amplified, for example, if the whole wizarding community turns against him unjustly. He might feel so alone and hopeless that he loses his will to live. Also, if he discovers some dark secrets about his past that make him question his own identity and worth, it could lead to this state.
Iruka - sensei could be one of those who help Naruto. He has always been like a father figure to Naruto and would offer emotional support. Rock Lee could also be a great helper. Lee, who has trained his body to be extremely powerful, could teach Naruto some physical combat techniques that don't rely on eyesight. And of course, the Nine - Tails inside Naruto might also play a role. It could potentially share some of its power or knowledge to help Naruto deal with his new situation.
One possible consequence could be a change in his fighting style. Without his eye, he might have to rely more on his other senses like hearing and smell, similar to how blind ninja in the Naruto universe adapt. For example, he could focus more on using his chakra to sense the movements of his opponents.
Naruto might also go through an identity crisis. His whole life has been centered around being a ninja with chakra and using powerful jutsu. Losing his chakra could make him question who he is and what his purpose is. He would have to redefine himself and find a new way to contribute to the ninja world. This could be a long and difficult process for him.
Maybe he starts to train with one - armed techniques. He could learn from other one - armed ninjas if there are any in the fanfiction world. His friends would also play a big role. They might encourage him to keep going or help him in his training.
A 'Naruto loses his memory fanfiction' might involve a mysterious curse placed on Naruto. He forgets not only his personal relationships but also his ninja skills. His sensei, Kakashi, could take it upon himself to reteach Naruto everything from the basics. Meanwhile, Naruto might form new, unexpected relationships with people in the village who don't know the 'old' him, and these new relationships could either help or hinder his journey to regain his memory.
If the Kyuubi loses its memory in the fanfiction, it could potentially change the entire Naruto storyline. For instance, Naruto may no longer have the same source of power that he once had. Instead, he might have to rely on his own natural talents and training more than ever. The Kyuubi, without its memories, could start to develop a new personality. It might become more curious and innocent, which would be a huge contrast to its previous menacing self. This would force Naruto to adapt and also give the other characters in the series new ways to interact with both Naruto and the Kyuubi.
Maybe Orochimaru manages to get his hands on Naruto and subjects him to some dark and twisted experiments. These experiments could mess with Naruto's mind, causing him to lose his sanity. He could start seeing illusions all the time, believing his friends are his enemies. As a result, he'd be in a constant state of confusion and aggression, and his once - positive attitude towards life and ninja - hood would be completely gone.
Maybe he loses his humanity as a result of a failed experiment. Some rogue ninja or a secret organization might try to enhance his power but end up corrupting his very essence. As a consequence, Naruto could start to show no mercy, lack empathy, and use his powers for destruction rather than protection.