There are several potential causes. Perhaps the Beast's temper gets the best of him for a moment and he accidentally causes her harm, although he would be extremely regretful afterwards. Or, there could be a new, malevolent character in the fanfiction, like a dark spirit that lurks in the forest around the castle and attacks Belle when she ventures out. It could also be that she was trying to help the Beast with something related to his curse and something went wrong, resulting in her getting hurt.
A mysterious new enemy that has infiltrated the village could be the cause. This enemy might be on a mission to disrupt the balance in the ninja world and sees Sarada as an obstacle. They could attack her when she is alone or on a solo mission. Their motives could be related to some ancient grudge against her family or the village as a whole.
There are also the possibility of some inter - dimensional beings. In the Ben 10 universe, there are many different dimensions. If a being from another dimension comes through with the intention of causing chaos, they could target Ben. They might have powers that are completely foreign to Ben and his allies, making it difficult for him to defend against them. And let's not forget about the possibility of some of Ben's old enemies teaming up. If they work together, they could come up with a plan to really hurt Ben, whether it's physically or by ruining his reputation among the people he protects.
There could be a new character in the fanfiction, like a bully who targets Lincoln just because he seems like an easy target and ends up hurting him.
It could be a new villain that has emerged. This villain might have some sort of vendetta against Steven specifically, perhaps because of his unique powers or his role in the gem world.
Perhaps it's a mysterious figure in the fanfiction world. This unknown character could have some ulterior motives against Jacob and find ways to hurt him, like setting traps for him.
It could also be a new, unknown enemy. Maybe there's a secret organization that emerges in the fanfiction that has a vendetta against those with electric powers like Michael. They could target him specifically and cause him harm.
The Beast would likely be extremely worried and angry at himself. He might immediately rush to her side, blaming his own cursed existence for putting her in danger. His first instinct would be to protect her and find a way to ease her pain.
Belle could get sick due to the drafty corridors of the castle. The Beast would be beside himself with worry. He might sit by her bedside all day, reading to her from his favorite books in his attempt to comfort her. As she recovers, she might find new aspects of his personality that she admires.
Even some of the school faculty could be responsible in a sense. For instance, a strict teacher who doesn't understand Danny's situation might push him too hard during detention or some other punishment, which could lead to an accidental injury. Or a science teacher's experiment could go wrong and Danny could get caught in the crossfire.
This idea could lead to exploring how the Beast's perception and relationship with Belle might change. Maybe it would make their connection more profound as it wouldn't be based on outward appearance.