Maybe he got injured during a dangerous adventure. For example, he could have fallen from a great height while chasing a dark wizard and broken his legs.
It could be the result of a curse. In the fanfic world, there are many powerful and evil curses. One of them might have been cast on Harry, affecting his ability to walk. Say, a curse that slowly drains his physical strength and mobility.
Another possibility is that he's suffering from a magical illness. There could be some rare magical diseases in the fanfic's universe that target the nervous system or the muscles, thus preventing Harry from walking.
One possible reason could be that he was injured during a battle with dark wizards. Maybe he was hit by a powerful curse that affected his legs. Another reason might be related to some kind of potion gone wrong. For example, if he accidentally drank a potion that had negative side effects on his motor functions.
Well, his friends might rally around him. Hermione would probably start researching every possible spell or potion to help him. Ron, on the other hand, would be there to offer moral support and practical help like getting Harry around. For instance, he could use a levitation charm to move Harry from place to place.
One possible reason could be that he was severely injured in a battle. Maybe he got his legs broken or had some internal damage that affected his ability to move them. For example, if he fought against a really powerful enemy who used a jutsu specifically targeting his legs, it could leave him unable to walk.
One possible reason could be that the magic used against him is of a very dark and ancient nature that defies the normal healing spells. For example, if it's a curse created by a long - forgotten dark wizard with unique and unbreakable properties.
One possible reason could be poverty. If the girl comes from a very poor family, she might not have proper footwear. Another reason could be some form of punishment. Maybe she did something wrong according to the rules of a particular institution or group. However, this is really cruel and unfair treatment for a girl.
In the 'Harry Potter can't walk fanfiction' when Harry loses his ability to walk, the story could develop in a very emotional way. His friends and allies would be determined to find a way to restore his ability to walk. Meanwhile, Harry himself might start to see the wizarding world from a different perspective. He may notice things that he overlooked before when he was able - bodied. He could also become a symbol of perseverance for other students who are facing difficulties. The story could also involve Harry learning to use magic in new ways to compensate for his lack of mobility, such as using telekinesis spells more effectively.
It could be that he failed at something important like a difficult task or a competition within the story of the fanfic. This would surely make him feel down.
One reason could be that the fanfic writer portrays Ron as overly jealous of Harry. In the original books, Ron has moments of jealousy but not to an extreme. If in the fanfic it's overdone, Harry might not be able to stand him. For example, if Ron is constantly trying to undermine Harry's achievements out of jealousy.
Fanfiction can sometimes deviate a lot from the original story. It might be that the official creators or those in charge of the Harry Potter brand don't want the characters to be misrepresented in fan - made stories, so they restrict access to it for Harry or anyone in the context of that world. Another aspect is that some fanfiction may have a negative or unethical undertone, and to protect the integrity of the original story, access is prohibited.
In the fanfiction's world - building, there could be a cultural or religious group in the wizarding community that Harry has become a part of, and they have a strict no - meat diet. This newfound identity or connection has led to him not being able to eat meat.