One possibility could be that after facing too many losses, like the deaths of his friends in the fight against Voldemort. For example, if Hermione and Ron were killed right in front of him during a final battle. Harry might feel that all his efforts were in vain and lose the will to live. Another reason could be that he discovers some dark secrets about the wizarding world that shatter his beliefs. Maybe he finds out that Dumbledore had some truly evil intentions all along and that everything he fought for was based on a lie.
In the fanfiction, it could be related to his relationship with Ginny or other important people in his life. If Ginny left him for some reason, like being under a spell or falling in love with someone else because of some magical interference. Harry, who has always been through so much, might see this as the final straw. His heartbreak, combined with all the past burdens, could make him lose the will to live. He might feel like there's nothing left for him in a world where he can't even have a happy relationship with the person he loves.
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.
It could be Voldemort, even if he's already been defeated in the original story. His lingering influence might still haunt Harry. Maybe there are some dark magic remnants of Voldemort that keep causing problems and making Harry feel hopeless.
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.
One possibility could be that Harry is cursed by a dark wizard. This curse slowly drains his magic, causing him to struggle with everyday wizarding tasks. He might have to rely more on his non - magical skills and the help of his friends. For example, Hermione could use her vast knowledge to find a way to reverse the curse while Ron provides emotional support.
He would become more humble. Losing his magic would make him realize that he's not invincible. He'd start to value the people around him more. For instance, he'd be more grateful for Hermione's intelligence and Ron's loyalty.
One possible consequence is that he might get injured more easily. Without his glasses, his vision is impaired, and being blindfolded makes it even worse. He could trip over things, bump into walls or other objects, and get bruises or cuts.
In this fanfiction, it could be that the pressure of being 'The Chosen One' finally gets to Harry. All the battles, the losses he has endured, and the constant threat of Voldemort might push him over the edge. He could start acting erratically, pushing away his friends and allies because he no longer trusts his own judgment.
Well, in the 'Harry Potter loses his magic fanfiction', he could end up finding new strengths within himself. Maybe he discovers that his courage and intelligence are enough to get him through tough situations even without magic. He could also start to see the wizarding world from a different perspective, one of the non - magical people, and perhaps work towards making the two worlds more inclusive.