What happened to Percy’s mom?

Percy's mother was disolved, and then kept frozen in the Underworld.

Then, with an angry roar, the monster closed his fists around my mother’s neck, and she dissolved before my eyes, melting into light, a shimmering golden form, as if she were a holographic projection. A blinding flash, and she was simply … gone.

A shimmering image hovered over the void: my mother, frozen at the moment she’d dissolved in a shower of gold. Her face was distorted with pain, as if the Minotaur were still squeezing her neck. Her eyes looked directly at me, pleading: Go!