Prince Paris of Troy did. She had been promised to him by Aphrodite in return for his giving her the prize apple in the beauty contest on Mount Ida. So Aphrodite helped Paris kidnap Helen.
She was Helen of Troy. So it was Troy. __Helen was NOT from Troy originally. She was kidnapped from Sparta, her husband Menelaus was the king of Sparta. Paris of Troy either kidnapped her, or she...
Paris because he wanted to marry Helen but King Menelaus asked her first so she said yes and then Paris kidnapped her because he loved her but he couldn't marry her because she was Engaged.