A cause of death has not been disclosed. Westwick, who played Chuck Bass in "Gossip Girl" opposite Trachtenberg's Georgina Sparks, mourned his former co-star by sharing a photo of her on his Instagram Story.

Kenan Thompson shared a touching tribute to his fellow Nickelodeon alum Michelle Trachtenberg, who died at age 39 on Wednesday, Feb. 26.

Kenan Thompson shared a sweet tribute to fellow Nickelodeon alum Michelle Trachtenberg, whose sudden death was announced on Feb. 26.

Rosie O'Donnell, Ed Westwick, SAG-AFTRA and others react to the 39-year-old's passing The post Michelle Trachtenberg’s Former Co-Stars Heartbroken Over Her Death: ‘I Wish I Could Have Helped’ appeared first on TheWrap.

The late actress was best known for her roles as Georgina Sparks on the CW series and Dawn Summers on Buffy the Vampire Slayer. Her storied career in entertainment began after she landed roles on Nickelodeon's The Adventures of Pete & Pete and soap series All My Children.

Michelle Trachtenberg's death Wednesday left her former co-stars like Blake Lively, Rosie O'Donnell, Melissa Gilbert and Kenan Thompson shocked and saddened.

Rosie O’Donnell, who starred alongside Trachtenberg in the starlet’s first movie, the Nickelodeon-produced 1996 film adaptation of the children’s novel “Harriet the Spy,” called the actress’ death “heartbreaking.”