Elena would likely be angry. She has her own agency and doesn't like being controlled. If Stefan kidnapped her, she would fight back. She might use her knowledge of the supernatural world and her relationships with other characters to try to escape. For example, she could try to call out to Damon telepathically if they have some sort of connection in the story.
One possibility is that Stefan is under some dark spell or influenced by an evil force. Maybe he kidnaps Elena to protect her from a greater danger that he believes only he can handle, but his actions are misinterpreted. For example, there could be a new vampire hunter in town who has a special weapon that can kill vampires instantly, and Stefan thinks the only way to keep Elena safe is to take her to a hidden location.
Stefan would be extremely worried and angry. As Elena's other love interest, he has a deep connection with her. He would immediately start looking for her, using all his vampire skills and resources. He would probably suspect Damon right away, given their history of rivalry over Elena.
She could be terrified at first. After all, Klaus is a very powerful and dangerous vampire. But Elena is also a strong - willed character. So she might quickly gather her courage and start looking for ways to escape.
The Salvatore brothers, Stefan and Damon, are the most likely candidates. They both have a strong connection to Elena and would stop at nothing to rescue her. Stefan's sense of chivalry and Damon's fierce protectiveness would drive them to track down Katherine and get Elena back.
I'm sorry, I haven't read this specific fanfiction. But you can try to find it on popular fanfiction websites like Archive of Our Own or Fanfiction.net.
One common theme is the idea of eternal love. Since they are in a supernatural world, their love is often depicted as something that can transcend time and death. For example, in many fanfictions, even if one of them is in danger or seemingly lost, their love remains unwavering.
Caroline, for example, might be more accepting as she is also a vampire. She could help Elena adjust to her new life. Damon, on the other hand, might be jealous or feel threatened that Stefan made Elena a vampire. It could change the power balance between the three of them.
One great fanfiction is 'A New Beginning for Stefan and Elena'. It explores a different path for their relationship after the events in the show, where they face new challenges together in a world filled with both old and new vampires. It's a really interesting take on their relationship and shows a more mature side of their love.