He might have used magic prosthetics. With the help of his friends like Hermione, who is brilliant at magic research, they could have found a way to create a magical arm that functions almost like a real one.
Harry could have initially been in deep despair. But then, he remembered all the times he had faced difficulties before. He might have started training himself to do things with just one arm, and also learned new spells that could be cast without the need of two hands. He was a determined person, after all. In the fanfiction, he may also have found support from his friends and the wizarding community, who admired his courage even more after this incident.
Well, in some fanfictions, it might be due to a dangerous experiment gone wrong. Harry could have been trying to create a new spell or potion to defeat Voldemort once and for all, but something exploded and cost him his arm.
He might first go into denial. He could keep trying to do the things he used to do with his powers, like swing from building to building, only to fall. Then he could start to rely on his intelligence more. Since Peter Parker is a smart guy, he'd use his scientific knowledge to try to find a way to get his powers back. He may also look for allies who can help him, like other superheroes or scientists.
Later, he could become depressed. Harry has always valued his friends highly, so losing them would be a huge blow. He might isolate himself in his dormitory, not participating in school activities as much. He would probably be constantly thinking about what went wrong and how to fix it.
Maybe he was protecting someone in Wave. He's always the self - sacrificing type, so he could have put himself in harm's way to save a villager or a friend.
He might turn to his studies more intensively. Since he has no friends to distract him, he could pour all his energy into learning new spells and magic, becoming more powerful in the process.
The world of Harry Potter is about magic, friendship, and heroism. Creating fanfiction around sexual topics is disrespectful to the source material and its fans. J.K. Rowling created a magical universe that doesn't need such inappropriate additions.
Losing his arm would be a huge blow to Harry. His friends' reactions would be mixed. Hermione would be practical, trying to find magical solutions or aids for Harry. Ron would be more emotional, showing his support through his presence. Some of his other friends might be a bit afraid or unsure at first. But over time, they would all come to accept Harry as he is now and their relationships would be based on a new level of understanding and support.
Maybe he was in a dangerous duel with a powerful dark wizard. The dark wizard could have cast a spell that was specifically designed to maim or disarm in the most literal sense.
Naruto might become more introspective. He has always been action - oriented, but losing his arm could force him to think more about his actions and their consequences. For example, he may be more cautious when engaging in battles.
Perhaps it's an old enemy that was thought to be defeated. This enemy might have been biding their time and finally finds a way to get revenge on Harry by driving him mad. It could be a Death Eater who was overlooked or a creature from the Forbidden Forest with a grudge.