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.
Well, one possibility is that Elena's friends would immediately start a search party. Damon, for instance, would be furious and stop at nothing to find her. He might enlist the help of Stefan and Bonnie. They could use Bonnie's magic to try and track Klaus' whereabouts.
One great Klaus and Elena fanfiction is 'The Bond Beyond Blood'. It explores a unique relationship between them where they have to team up against a common enemy. The story is filled with intense emotions and some great character development for both of them.
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.
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.
This type of fanfiction often dives into the psychological aspects of the characters. The kidnapped mate may experience a range of emotions such as fear, anger, and confusion at first. As the story progresses, they might start to see Klaus in a different light. The writer could also introduce other characters who react to this situation, either trying to rescue the mate or siding with Klaus, which adds more layers to the story.
One possible plot could be that Klaus believes Bonnie has the key to a powerful spell that could help him gain ultimate power. So he kidnaps her. He whisks her away to a remote island. There, he tries to persuade her to help him, using both threats and false promises. He might start off being cruel but then gradually start to have some strange respect for her as she resists him.
One possible plot could be that Kol kidnaps Elena to use her as bait for his enemies. He might think that by having Elena in his custody, her friends, especially the Salvatores, would come to rescue her and he could then ambush them. Another idea is that Kol has some sort of vendetta against Elena because of her connection to the doppelganger bloodline. So he kidnaps her to extract revenge on the entire line.