Michelle Trachtenberg's friends and costars react to the 'Buffy' and 'Gossip Girl' star's sudden death at 39: 'I wish I could have helped'

Michelle Trachtenberg played Sarah Michelle Gellar's sister on "Buffy the Vampire Slayer." Frazer Harrison/Getty Images Michelle Trachtenberg was found dead on Wednesday morning. She was 39. She began her acting career as a child star and is best known for "Buffy" and "Harriet the…