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.

Comedian and former Nickelodeon child star Kenan Thompson shared a video on Instagram of the two of them on Nick, writing: “Our first Nick movie star has departed us!! She was my friend and now she rests!!” One of Trachtenberg’s first roles was in the Nick series “The Adventures of Pete & Pete.”