Maybe there's a supernatural element involved. While locked in, Bella starts to sense a strange presence. She realizes that the school was built on an ancient burial ground and she has to find a way to escape before the entity gets to her. This could add to the scared factor and make for an interesting fanfiction.
One idea could be that Bella discovers a secret passage in the school while locked in. She's scared at first but her curiosity gets the better of her. As she explores, she uncovers some long - forgotten secrets about the school's history.
You could introduce some conflicts or mysteries. For example, strange noises coming from the attic or a locked room that she feels compelled to investigate. As for the plot, perhaps she finds a clue that might lead to her escape, but it's not easy. She has to face her fears and solve puzzles along the way.
Well, perhaps Bella is in a small town during a snow storm. She's staying at an inn and hears rumors of a ghost that haunts the town during such bad weather. As the snow storm intensifies, she starts to see strange shadows outside her window. She's so scared that she barricades herself in her room. Later, she finds out that it was just a prank by some local kids who like to play on the fears of tourists during storms.
A plot idea could be that Bella gets stuck outside during a thunderstorm. She's terrified, but then a stray dog comes to her side and stays with her until the storm passes. This event could lead to Bella adopting the dog and the dog becoming a source of comfort during future storms.
Well, in some fanfictions, Bella might be scared of Edward because of his vampiric nature. His superhuman strength and speed could be intimidating. Also, the unpredictability of his actions as a vampire who is constantly fighting his bloodlust could make Bella feel scared.
Maybe Aziraphale is scared when a powerful otherworldly entity shows up that he thinks might harm Crowley. So he tries to protect Crowley in his own bumbling way. Crowley, in turn, is both amused and touched. He then shows Aziraphale that he can handle himself just fine, but also thanks Aziraphale for his concern. They end up having a little adventure trying to figure out what this new entity wants.
One common theme could be redemption. If Bella is in a broken and scared state, she might be on a journey to redeem herself or find her place again. Another theme could be survival, as she tries to get through whatever has made her broken and scared.
One possible fanfiction could be that Bella is caught in a sudden and violent snow storm while on a hike in the mountains. She's scared as she realizes she's lost her way and her phone has no signal. The cold is seeping into her bones and she starts to hallucinate about strange creatures in the snow. But then she remembers some survival skills she learned before and starts to build a snow shelter to wait out the storm.
Perhaps it's a more romantic take. Bella could be locked in a room by a love interest who is too shy or conflicted to face her directly. So, in a rather clumsy way, they lock her in a room with the intention of finally confessing their feelings but are struggling to find the right words.
A secret organization could be the antagonist. They might see Bella as a threat to their hidden agenda. For instance, if she has stumbled upon some information about their illegal activities, they lock her in a room to keep her silent and prevent her from sharing what she knows with the outside world.
One common scenario is when Edward loses control. Maybe he gets too close to Bella when he's thirsty for blood, and she can see the struggle in his eyes. This nearness and the thought of him potentially harming her can be terrifying for her.