Perhaps in the fanfiction, Harry is kidnapped by a powerful magical creature that has a vendetta against wizards. When it grabs Harry, his glasses are knocked off. The creature takes him to its underground den. Harry, without his glasses, is at a great disadvantage as he can't see well. But he remembers the magic within him. He tries to communicate with the creature using Parseltongue (if it's a snake - like creature). He hopes to either convince it to let him go or at least find his glasses so he can better defend himself and find a way back to Hogwarts.
One possible plot could be that while kidnapped, Harry's lost glasses become a crucial element for his escape. Maybe the kidnappers don't realize the magical properties of the glasses and Harry uses a spell that is activated only when his glasses are in a certain state. For example, he could have a spell on them that when they are broken in a specific way, it sends a signal to his friends at Hogwarts.
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.
A fanfiction might have it that a new dark force in the wizarding world targets Harry. They manage to trap him in an old, abandoned manor. They torture him using various dark spells, and during the chaos, his glasses are lost. Harry endures the pain and tries to remember some of the spells Hermione taught him for self - defense. Eventually, he breaks free from his bindings and starts looking for his glasses. He stumbles around the manor, using his wand light to search, while also being on the lookout for the return of his captors.
One such fanfiction could be 'The Dark Trials of Harry Potter'. In this story, Harry endures intense torture by dark wizards which causes him to lose his glasses. It shows his struggle to survive without his vision aid while facing the evils. However, such fanfictions often deviate from the original J.K. Rowling's vision, but they can be interesting to explore different aspects of the characters.
One possible plot could be that a dark wizard kidnaps Harry Potter in order to steal his powers. In the chaos of the abduction, Harry loses his glasses. Without his glasses, he has trouble seeing clearly which adds to his vulnerability. The wizard might be working for Voldemort or could be a new enemy. His friends Hermione and Ron would then embark on a quest to rescue him. They would face many challenges along the way, like getting past dark magic traps and finding the location where Harry is being held captive.
One possible plot twist could be that without his glasses, Harry has a hard time recognizing his captors. It might turn out that they are people he thought he could trust, like some seemingly friendly Hufflepuffs who are actually under the Imperius Curse. This would add an element of betrayal and confusion to the story.
One possible plot could be that Daria is kidnapped by a group who mistake her for someone else. In the chaos of the kidnapping, her glasses are lost. As she's held captive, she has to rely on her wits more than ever without her glasses to see clearly. She might start to analyze her captors' behavior and motives, trying to find a way to escape. Maybe she uses her sharp tongue and intelligence to outwit them, all while dealing with the inconvenience of not having her glasses.
The plot could develop with the girl using her memory of the place she was kidnapped from to try and escape. Since she lost her glasses, she might recall details like the smell of the room, the texture of the floor, etc. This could lead her to find a hidden exit or a weapon to defend herself.
In this fanfiction scenario, the loss of her glasses during the kidnapping could lead to an unexpected alliance. Felicity, without her glasses and in a vulnerable state, meets another captive who helps her. Together, they figure out a way to escape. And when she is finally free, she discovers that the whole incident was related to a corporate espionage plot where her glasses were a key piece of evidence or a target because of some unique design or data they held.
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.