In the story, Percy's mother is imprisoned by Hades in the depths of Hell.
The text tells us that Percy's mother, Sally Jackson, first disappears after the Minotaur (Pasiphae's son) grabs her. We don't find out where Sally is until later in the story.
Accordingly, Sally is being held hostage in the Underworld by Hades. Hades thinks that Poseidon has stolen his Helm of Darkness. However, this isn't true. Ares, the god of war, is the one behind the loss of Zeus and Hades' symbols of power. It is Ares' intention to cause a war between Zeus, Poseidon, and Hades. So, Ares has Luke steal Zeus' master bolt. This causes conflict between Poseidon and Zeus, because the latter believes that his brother is behind the disappearance of his powerful bolt.
Additionally, Ares had originally planned to plant Zeus' master bolt in Hades' possession. In Ares' mind, this would cause conflict between Zeus and Hades. So, Ares is mainly interested in instigating a war between the three most powerful gods: Zeus, Poseidon, and Hades.
Meanwhile, in Chapter 19, Hades accuses Percy of helping Poseidon steal his helm of darkness. The god of the underworld thinks that Poseidon is just trying to start a war. Because Hades thinks that Percy is helping Poseidon, he kidnaps Sally (Percy's mother). Hades holds Sally in the underworld as a bargaining chip. For his part, Percy tells Hades that he has gotten his facts all wrong. He maintains that he did not steal Hades' helm of darkness.
However, Hades does not believe Percy. He tells Percy that he will only let Sally go after his helm is returned. Since Percy does not have the helm, he cannot return it. So, Percy is forced to leave Hades. In Chapter 20, Percy battles Ares and is able to retrieve the Helm of Darkness. Satisfied that he has his helm back, Hades returns Sally to Manhattan.