Well, in such a fanfiction, Harry might disappear to train in some hidden place, away from the prying eyes at Hogwarts. Maybe he goes to a secret location with a powerful wizard who teaches him advanced magic that he couldn't learn at school. This could include dark magic defenses or ancient spells.
In fanfiction, if Harry Potter disappears, it could lead to chaos at Hogwarts. His friends like Hermione and Ron would likely be on a desperate search for him. They might suspect the Dark forces at work again, and start investigating all the possible places where Harry could have been taken or hidden.
It could be Dumbledore. Even if he's dead in the original story, in fanfiction, anything is possible. He might have left behind some kind of magic that allows him to train Harry from beyond the grave, or he could have faked his death to take Harry to a secret location for training.
The setting in which Harry disappears and returns also matters. If it's in the middle of Hogwarts, the school would be in chaos. Teachers would be trying to keep the students calm while secretly searching for Harry. And when he returns, the reactions of different houses - Gryffindor being relieved and proud, Slytherin perhaps being suspicious - could add more layers to the story.
Perhaps Harry is kidnapped by a mysterious and powerful magical being. This being has the ability to make Harry disappear without a trace. His friends then embark on a dangerous quest to find him, facing many magical challenges along the way. They might have to search in hidden and dangerous parts of the wizarding world that they have never been to before.
He might end up in a place where he can't see clearly without his glasses and struggles to find his way. For example, he could be in a dark forest where he stumbles around, trying to figure out where he is and how to get back. Without his glasses, his magical abilities might also be affected in some way, like his spell - casting accuracy.
The wizarding world would become a much darker place. Voldemort would rule with an iron fist. There would be no more 'Boy Who Lived' to give people hope. The streets that were once filled with the joy of magic would be filled with fear. The Ministry of Magic would enforce Voldemort's cruel laws, and any form of dissent would be severely punished.
The theme of sacrifice could also be present. Maybe Harry disappeared because he sacrificed himself for a greater good. And his return could be a sort of resurrection, where he has to deal with the consequences of his sacrifice and how it has affected the wizarding world.
Voldemort could be the antagonist. Even if he was defeated in the original story, there could be a scenario where some remnant of his dark magic causes Harry and Hermione to disappear. Maybe he had a secret plan that was set to activate after his death and it involved taking them out of the picture.
It could be a new dark wizard who has been secretly plotting in the shadows. This wizard might be jealous of Harry's fame and power, and decides to make him disappear so that he can rise to power unopposed. He could have a group of followers who are experts in dark magic and are able to carry out his plan to make Harry vanish.