In the fanfic, it could be related to an emotional turmoil as well. Adrien has a lot of personal issues and secrets. If something really bad happened, like he found out some truth about his family that shocked him to the core, it might have made him distracted during a fight or a dangerous situation. This distraction could have led to him being in the wrong place at the wrong time and getting caught under the rubble when something like an explosion or building collapse occurred.
Perhaps he is found by other characters like his friends or his kwami. His kwami could try to give him some extra strength to recover. If his friends find him, they would call for help and stay with him until medical assistance arrives. They might also try to keep his identity as Chat Noir a secret if he was in his superhero form when he collapsed.
Well, in the fanfic, Adrien could be in the rubble hospital due to a battle gone wrong. Maybe he got seriously injured while fighting an akuma. The hospital setting might be a place for him to have some self - reflection and also for his relationship with Ladybug to develop further, like she could visit him secretly.
The rubble hospital might be a place of danger for Adrien. If Hawk Moth's minions find out he's there, they could try to finish him off. It adds an element of suspense to the fanfic.
One possible reason could be that he got injured during a fight with an akumatized villain. In the Miraculous Ladybug universe, battles can be quite intense and Adrien, as Chat Noir, often puts himself in harm's way to protect Paris. Another reason might be some sort of illness that was magically induced, perhaps by a new villain with a different kind of power that targets the heroes in a non - traditional way. It could also be due to overexertion from using his Cat Miraculous too much without proper rest, which could lead to his body breaking down and him ending up in the hospital.
Maybe he was poisoned. There could be a villain with the power to create toxic substances, and Adrien accidentally got exposed. It could also be related to his double life stress. Constantly juggling between being Adrien and Chat Noir might take a toll on his health, leading to a collapse or some kind of health issue that requires hospitalization.
It could also be a case of misinformation in the fanfic. Adrien might think Ladybug has turned evil or is about to cause great harm. So, in his attempt to save the city or his loved ones, he makes the tragic decision to kill her, believing it's for the greater good.
It could be related to his emotional state. Adrien has a lot of pressure on him, both as a model and as Chat Noir. The stress could have taken a toll on his health, causing him to collapse and be hospitalized. In the fanfiction, this could be the starting point for his friends, like Nino and Alya, to realize how much Adrien has been hiding and struggling, and they could come together to support him in a new way.
This event in the fanfic could also lead to new characters emerging. Maybe someone would try to take on Ladybug's mantle in a different way. Or new villains could rise, seeing the opportunity in the chaos. The magic of the Miraculous might be affected too. Since Ladybug was an important part of the magical balance, her death could have unforeseen consequences on the powers and the Miraculous themselves.
In such a fanfiction, one possible reason could be the intense pressure he faces as a model and son. Constantly having to meet high expectations and having little personal freedom might drive him to despair. For example, his father's strict control over his life and his public image could make him feel trapped.
A plot could involve Adrien collapsing injured as a result of trying to protect Ladybug. There could be a powerful akuma that has the ability to cause harm just by being close. Adrien, in his selfless nature, jumps in front of Ladybug to shield her. He then collapses due to the akuma's power, and Ladybug has to find a way to both cure the akuma and heal Adrien. This would show the deep bond between them and Adrien's heroic nature.