It could also be due to a spell or an akuma's special ability. In the Miraculous universe, there are many powers that can immobilize characters. For example, if an akuma had the power to paralyze people, Adrien might have been caught in it. And since he's often in the middle of the action, he'd be a prime target. He might be motionless as his body is fighting off the effects of this external force, trying to break free from it so he can transform into Chat Noir and save the day.
Well, it could be that he's been injured in a battle against an akuma. Maybe the akuma had a particularly powerful attack that left him unconscious and motionless.
One reason could be that he's in a state of shock. Maybe he witnessed something really traumatic that made him unresponsive. For example, if he saw Ladybug in a life - threatening situation and couldn't do anything to help immediately, it might have sent him into a state of mental paralysis.
It can create a sense of suspense. The other characters will be worried about him and the readers will be eager to know what will happen next. For example, if Ladybug sees him motionless, she'll try to figure out what's wrong and how to help him, which drives the plot forward as she investigates.
Ladybug might come to his rescue. She could use her Lucky Charm to find a way to heal him or break whatever spell he's under.
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.
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.
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.
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.
One possible reason could be that Marinette has been acting strangely in ways that seem to Adrien to be related to Hawkmoth. Maybe she has been seen in places or at times that are suspicious, like near akuma attacks or where Hawkmoth might be operating from. Another reason could be that someone has been spreading false rumors about Marinette, leading Adrien to believe she has some connection to Hawkmoth.
One possible reason could be that Adrien is being blackmailed by Hawkmoth. Maybe Hawkmoth has some information or has threatened someone Adrien cares about deeply, like his friends or family. So, in order to protect them, he decides to help Hawkmoth, even though he knows it's wrong.