Another reason might be related to a new magical artifact or potion. Harry might have come across something that had an unforeseen effect on his glasses. It could be that a new charmed item he was using interacted with his glasses and caused them to disappear.
There could be a fanfiction where Harry is studying in the library with his broken glasses. He's squinting hard to read the old, faded texts. Hermione notices and offers to help him fix his glasses with a spell she's been researching. But Harry refuses at first, wanting to do it on his own. This creates a bit of tension between them, and as they argue, Ron comes in and tries to mediate. It could be a comical and character - driven fanfiction scenario.
A fanfiction could start with Harry's glasses being stolen by a mischievous house - elf. Harry starts to squint to see in his classes, which causes some problems. For instance, in Potions class, he misreads the instructions and creates a minor explosion. He spends the day trying to find the house - elf and his glasses. Along the way, he meets some other characters who help him in unexpected ways. By the end of the day, he gets his glasses back and has learned a valuable lesson about not taking his eyesight for granted.
One possible reason could be as a form of humiliation. His enemies might take his glasses to make him feel vulnerable and less in control. Without his glasses, Harry is severely visually impaired, so it's a way to undermine his confidence.
A simple plot could be that Harry's glasses break right before an important exam. He has to squint to read the questions, which makes it really difficult for him. His friends try to help him, but the examiner doesn't allow any outside assistance. Harry has to dig deep and rely on his memory of the material to get through the exam. In the end, he passes, and it shows his determination.
One reason could be to escape danger. Maybe Hogwarts is facing a new and extremely powerful dark threat that even Dumbledore can't fully protect the students from. Another reason might be for a unique educational opportunity. There could be a school in another part of the wizarding world that offers a specialized curriculum in, say, ancient magic or magical creature training that Harry is interested in. Also, it could be due to a conflict with certain students or teachers at Hogwarts. If there are people constantly causing trouble for Harry, he might consider changing schools to have a more peaceful learning environment.
One reason could be that he wants to escape the constant danger at Hogwarts. Maybe he feels that Voldemort's influence is too strong there and he needs a fresh start. Another possibility is that he has a falling - out with some of the teachers or students, and he decides that a new school environment would be better for him.
He could misidentify something important. For example, he might think an approaching friendly ship is an enemy vessel because he can't see clearly. This could lead to a lot of chaos and misunderstanding among his comrades on the ship.
A possible plot could be that Harry is framed and sent to Azkaban. Without his glasses, he struggles to see clearly. One day, he finds an old magical object in his cell. He tries to figure out what it is by using his other senses. As he explores it, he starts to unlock some of its secrets which could be the key to his escape or a way to expose the real culprits behind his false imprisonment.
One possible fanfiction could be about Harry having trouble with his glasses after a spell goes wrong. He has to squint constantly to see clearly, and this leads to some comical situations at Hogwarts. For example, he mistakes a cat for a Kneazle during Care of Magical Creatures class.
One possible fanfiction could be that Harry loses his glasses during a Quidditch match. He has to squint to see the Snitch, which leads to some comical near - misses and some very frustrated team - mates. In the end, he manages to find his glasses just in time to catch the Snitch and win the match.