Voldemort or his Death Eaters are likely antagonists. They have always been out to get Harry, and chaining him up without his glasses would be a cruel and effective way to subdue him.
It could be a situation where Harry has been wrongly accused. Some overzealous magical law enforcers chain him up and confiscate his glasses as a form of punishment before a proper trial. This would create a lot of internal conflict within Harry, feeling betrayed by the very system he has been fighting for.
It could be the kidnapper. They might pick up the glasses as a way to taunt Felicity, or perhaps they keep them as a sort of trophy. It would add to the psychological warfare between them.
Well, one possibility is that it could lead to some comical moments. Harry might be fumbling around trying to find his glasses while the Snatchers are closing in, which could add a bit of humor to an otherwise tense situation. Maybe he accidentally bumps into things or misidentifies Snatchers because he can't see clearly.
Maybe she was startled by something sudden, like Thor using his powers or a loud noise. In many fanfictions, characters can be caught off - guard in such situations, causing their accessories like glasses to fall off.
In fanfiction, if Daria's glasses fall off during a kidnapping, it could add an element of vulnerability. Maybe she loses her main way of clearly seeing the situation and the kidnapper, which could heighten the sense of danger. For example, she might misjudge distances when trying to escape or not be able to recognize important details about her surroundings.
Well, it could be that the girl is really embarrassed. She might quickly try to retrieve her glasses while fumbling nervously. And this could attract the attention of some of the more mean - spirited gym - goers who might snicker, but then a group of nice girls could come to her defense and they all end up becoming friends.
Another gym - goer who has a soft spot for clumsy moments might be the one to help. This person could be someone who has had their own embarrassing moments at the gym and feels empathetic towards the girl. They might not only pick up the glasses but also share their own story to make the girl feel better.