One popular 'Harry Missing, Live' Potter fanfiction could be 'The Lost Boy Returns'. It focuses on the mystery of Harry's disappearance and the chaos it causes in the wizarding world. The characters are well - written, and it has an engaging plot that keeps you hooked from start to finish.
In the fanfiction, Harry Potter might live by himself because he is tired of the spotlight. After being in the public eye for so long, he desires a private life. He may also feel that living alone allows him to better understand himself and his powers. It could be that he has a prophecy or a personal quest that requires solitude. For example, he might be trying to master a very difficult and dangerous spell that he doesn't want anyone else to be in danger from if something goes wrong. Also, he could be exploring some dark corners of magic that the wizarding world might not approve of, and living alone gives him the freedom to do so without judgment.
I'm not sure specifically as there could be many different takes in such a fanfiction. It might involve Harry and his mom having some sort of unique adventure while living in a trunk. Maybe they discover a hidden magical world within the trunk or use it as a refuge from the dark forces in the wizarding world.
First, immerse yourself in the wizarding world by rereading the books and rewatching the movies often. Decorate your living space with Harry Potter memorabilia like wands, Hogwarts house banners, and Marauder's Map replicas. Join online fan communities to discuss theories, fanfic ideas, and share your love for the series. Try to incorporate magical elements into your daily life, like using a quill pen instead of a regular one for writing your own fanfic stories.
One possible reason could be the continuous loss of his loved ones. In the original series, Harry has already endured so much pain from losing Sirius, Dumbledore, etc. In the fanfiction, perhaps there are additional deaths or betrayals that push him over the edge. Another reason might be the overwhelming burden of being 'the Chosen One'. The constant expectation to save the wizarding world could finally break him.
Well, it could be when you cast a special spell that summons him or when there's a magical convergence that brings him to your world. But it's all up to your imagination and the story you want to tell.
One reason could be the extreme hardships the characters face in the original story. For example, Harry has lost his parents, faced Voldemort numerous times, and seen many friends die. In fanfiction, this could be amplified to a point where a character feels they have no will to live. Another aspect could be related to the sense of isolation. Some characters, like Snape, were always sort of alone in their struggles, and in fanfiction, this could lead to a loss of the will to live.
A new dark wizard or witch could be introduced as the antagonist. This character could be even more powerful and cruel than Voldemort. They could have a special power that allows them to break Harry's spirit, perhaps by constantly torturing his mind or by causing harm to those he loves in ways that are even more insidious than Voldemort's methods.
One possible theme could be extreme despair. In the Harry Potter series, Harry faces many challenges, but in this fanfiction, perhaps he's pushed to a point where he loses all hope. Maybe he's faced with a loss that he can't bear, like the death of all his friends or the destruction of Hogwarts beyond repair.
It could be Voldemort. Even if he's defeated, his actions might have left such deep scars on Harry. For example, all the people he killed and the destruction he caused. Harry might feel responsible for not being able to prevent more deaths and this guilt could make him lose the will to live.