One possible reason could be the overwhelming burden of his destiny. Constantly facing dark forces and the pressure of saving the wizarding world might have worn him down. Another reason could be the loss of his loved ones. For example, if in the fanfiction Hermione or Ron were killed, that could shatter his spirit.
One possible reason could be the continuous loss of his loved ones. In the original series, Harry has already endured so much pain from losing Sirius, Dumbledore, etc. In the fanfiction, perhaps there are additional deaths or betrayals that push him over the edge. Another reason might be the overwhelming burden of being 'the Chosen One'. The constant expectation to save the wizarding world could finally break him.
A new discovery could be the key. Maybe Harry stumbles upon a long - lost magic that has the power to reverse all the bad that has happened. This could give him a new sense of purpose and reignite his will to live. Also, if a new character enters the story, a wise old wizard perhaps, who offers him guidance and hope, it could lead to a positive turn.
The story could develop with Harry's friends trying to bring him back from the brink. They might search for a way to reverse whatever has made him lose his will to live. For example, if it's a curse, they could look for a counter - curse. Another way could be that Harry has a moment of self - realization and decides to fight back against the despair. Maybe he has a vision or a talk with a ghost from his past that gives him a new perspective.
He could find a new purpose. Maybe he starts to focus on helping the younger generation of wizards, teaching them how to defend themselves. This gives him a sense of hope for the future and reignites his will to live.
In this kind of fanfiction, Harry might feel extremely lost and depressed. He has been defined by his magic for so long, and without it, he'd have to re - evaluate his identity. Maybe he'd start to rely more on his friends, like Ron and Hermione, in different ways. For example, instead of using magic to solve problems, he'd have to use his wits and their combined non - magical skills.
The plot could progress with Harry using magic to compensate for his lack of glasses. For example, he could use a spell to make his eyesight better. Or, his friends could use their skills to help him. Hermione might use a spell to locate his glasses in the chaos of the brawl.
In such a fanfiction, Harry might be at a serious disadvantage. Without his glasses, he'd struggle to see clearly. His opponents could easily take advantage of his impaired vision. He might have to rely more on his other senses, like his hearing or his magical intuition. Maybe he'd cast a spell to enhance his vision temporarily, but it could backfire or be disrupted during the brawl.
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.
The story could develop with Harry trying to find a way to get his magic back. He might go on a quest, searching for ancient spells or magical creatures that could help him. For instance, he could seek out the Phoenix, known for its magical healing abilities.