Your best friend was a Crystal Gem and was going against White Diamond. As your friend was battling White Diamond with other Crystal Gems, he tells you to run, but while you're running, White Diamond picks you up and holds you in her hands.

"Ha, you're coming with me,"She says, looking down at your friend. "Good luck trying to get your friend, dumbass."

She then disappears with you. When she disappears with you, you appear in White Diamond's domain. She has kidnapped you.