Perhaps with his glasses, Harry would notice some small details that are crucial in the wizarding world. He might find hidden clues in the Hogwarts castle that would help him in his adventures against Voldemort. It could also change how he perceives the magic around him, as he would be seeing everything in a more accurate and detailed way.
In this fanfiction, it could be that Harry Potter was in a chaotic situation and simply misplaced his glasses. Maybe he was in a battle or a confusing adventure.
One possibility could be that in this fanfiction, Harry's glasses are some sort of magical anchor. Without them, he might accidentally slip into a different dimension. Maybe he was in the middle of a battle at Hogwarts, his glasses got knocked off, and suddenly he vanished. It could be that the glasses were enchanted by Dumbledore long ago to keep Harry in the 'right' magical space, and without them, he's lost in a sort of limbo.
Maybe it's a jealous student from Hogwarts. Someone who is envious of Harry's fame and power. This student has been studying dark magic in secret and decides to kidnap Harry to prove that they are better. They take his glasses to humiliate him as well. They might think that without his glasses, Harry is just an ordinary wizard and they can easily overpower him.
He might start by retracing his steps. He would think about all the places he had been recently and search those areas thoroughly. For example, if he was in the library before losing his glasses, he would go back to the table he was sitting at and look around on the floor and on the bookshelves.
One big challenge would be in his studies. Magic often requires precise wand movements and seeing spells being cast by others clearly. Without his glasses, he could misinterpret what a teacher is demonstrating, and this could affect his learning. Moreover, in Quidditch, he would have a hard time seeing the Snitch or other players clearly, which is crucial for the game.
Well, if the Snatchers made Harry lose his glasses, it could change the whole dynamic of his character. Harry is known for his glasses as part of his identity. Without them, he'd be in a rather difficult position. He might be more likely to be caught by the Snatchers as he wouldn't be able to see clearly to escape. It could also lead to some character development where he has to overcome this new obstacle, perhaps learning to trust his instincts more than his sight.
One possible fanfiction could be set during a fight with some dark wizards in a back alley. Harry is outnumbered and gets beaten up badly. In the chaos, his glasses are knocked off and broken. He has to rely on his other senses and magic to fend off the attackers until help arrives.