Well, perhaps Harry could be captured by a group of rogue Death Eaters who still believe in Voldemort's ideology. They torture him to find out if there are any remaining secrets of the Order of the Phoenix. In the process, his glasses get broken or lost. Harry then has to rely on his other senses and magic to try and escape. This could lead to a story of his inner strength and how he overcomes the physical handicap of not having his glasses.
Maybe Harry is lured into a trap by a powerful dark wizard who wants to use Harry's unique magic for his own evil purposes. Once captured, he is tortured. The loss of his glasses could symbolize his loss of the familiar and safe world he knew. He might then meet a new character, perhaps a fellow prisoner or a mysterious being who helps him get through the ordeal, and together they try to break free while Harry also has to deal with his impaired vision.
Well, it could start with Harry being lured into a trap by some mysterious enemies. They might be after the secrets of Dumbledore's Army or something. Once trapped, they start torturing him. His glasses could be lost when he's thrown around during the torture. Then, he might use his magical skills, like his ability to communicate with snakes (which he has from being a Parselmouth), to call for help or find a way to escape. He'd be at a disadvantage without his glasses, but his determination would keep him going.
In the fanfiction, when Harry Potter loses his glasses, it could lead to some really interesting character development. For example, his friends might have to step up and protect him more. Also, it could be a chance for him to discover new magical ways to improve his vision without the glasses, perhaps a spell or a potion that he has to search for.
His friends, Hermione and Ron, would surely help. Hermione with her intelligence and Ron with his loyalty.
It could be a group of rogue Death Eaters who think that kidnapping Harry would bring them back into Voldemort's good graces. They might take his glasses as a way to humiliate him, seeing Harry as the symbol of their defeat. They underestimate Harry's resourcefulness even without his glasses.
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.
Well, maybe he stumbles around blindly for a while, bumping into things and getting into all sorts of comical situations. He could also rely on his friends more to help him get around until he finds his glasses.
I'm not aware of any extremely famous fanfiction with that exact plot. There are thousands of Harry Potter fanfictions out there, but this seems like a rather specific and somewhat dark concept that may not be as widespread in well - known fanfictions. However, it's possible that there are some niche or more adult - themed fanfictions with this idea, but I can't recommend those as they might not be appropriate.
A plot twist could be that Harry's kidnapping and loss of glasses is the result of a time - travel mishap. A future version of himself (or someone else) accidentally causes the present - day Harry to be kidnapped and lose his glasses. The future version is trying to change the course of events for the better but ends up creating chaos in the present. Harry, without his glasses, has to figure out what's going on and how to fix the time - line while also escaping from his kidnapper. Another twist could be that the kidnapper is actually an undercover Auror gone rogue who is using Harry as a bait to draw out a much larger and more dangerous criminal organization. Harry's glasses are lost in the process, and he has to figure out who to trust among his captors.
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 a fanfiction, if Harry Potter loses his glasses, the plot will likely take a different turn. His lack of clear vision could make him seem less confident, which would change how others perceive him. Enemies might underestimate him, giving him an opportunity to surprise them. His search for a replacement or a solution to his vision problem could introduce new characters, like an old wizard who specializes in eye - related magic, and new sub - plots could unfold as a result.