She was caged by Bowser.

As Princess Peach sat in a dank dungeon, surrounded by metal bars, trapped in a portable cage that Bowser, her captor, was easily able to lift, she ruminated on the whole situation. Despite the media's tendency to exaggerate how often something like this had happened, this was, if she was counting correctly, the twelfth time the koopa had kidnapped her. More often than not, she tried to exact whatever petty vengeances she could while waiting for Mario's rescue, but that day was different. She felt, for a reason she couldn't fathom, like things were going to be, needed to be, different this time.