Blake Lively, Sarah Michelle Gellar, David Boreanaz, Ed Westwick, Kim Cattrall, Kenan Thompson, Melissa Joan Hart and more shared tributes to Trachtenberg.

Hollywood is grieving the unexpected loss of Michelle Trachtenberg, including several of the actress’ co-stars.

Trachtenberg — beloved for her roles in TV shows Buffy the Vampire Slayer, where she played Dawn Summers, and Gossip Girl, where she played Georgina Sparks — was declared dead on Wednesday at age 39. “It is with great sadness to confirm that Michelle Trachtenberg has passed away. The family requests privacy for their loss. There are no further details at this time,” the actress’ rep confirmed to The Hollywood Reporter. 

Trachtenberg was found dead in her New York City apartment by her mother on Wednesday morning, according to ABC News. The outlet reported that she is believed to have died from natural causes after experiencing complications from a liver transplant procedure.

Trachtenberg started her career as a child star. Her first feature film was at 10 years old in 1996’s Harriet the Spy, where she starred alongside Rosie O’Donnell, who shared a statement with People: “I loved her very much. She struggled the last few years. I wish I could have helped.”Harriet the Spy‘s director, Bronwen Hughes, opened up to the THR on Wednesday about her memories with the actress: “Michelle was like a lightning bolt of a casting discovery. She had fire, she had spark, she had truth and she had Harriet’s conviction, like her sense of self. No one else came in the room like that. So she had that acting gene that you can’t really teach. It’s innate, which is not something you find very often,” she said. “She had the ‘it’ factor, for sure.”

Trachtenberg was 10 when she made her feature debut as aspiring spy Harriet M. Welsch in the 1996 movie. She was also known for Disney’s Ice Princess, EuroTrip, Mysterious Skin, Black Christmas and 17 Again.

Many of Trachtenberg’s former co-stars and others took to social media to share their condolences, including Kim Cattrall, whom she worked with in 2005’s Ice Princess, and her 17 Again co-star, Leslie Mann shared on her Instagram Story: “I will miss you, Michelle.”Blake Lively, Chace Crawford, Taylor Momsen, Jessica Szohr in Gossip Girl along with the show’s co-creator Josh Schwartz, who took to his Instagram Story to write: “RIP Michelle. Thank you for Georgina. You will be missed but not forgotten.”

Lively took to her Instagram Story to share a photo of the first day she met Trachtenberg. “She was electricity,” she wrote. “You knew when she entered a room because the vibration changed. Everything she did, she did 200%. She laughed the fullest at someone’s joke, she faced authority head on when she felt something was wrong, she cared deeply about her work, she was proud to be a part of this community and industry as painful as it could be sometimes, she was fiercely loyal to her friends and brave for those she loved, she was big and bold and distinctly herself. And she always had yummy caramel smelling lipgloss on because she didn’t just want to sparkle on camera, she liked creating a nice experience for anyone who was in her orbit, even down to the subtle smell of her lipgloss because she cared about the sweet details.”

She continued in her tribute, “She was a kind person, through and through. Time passes. You take for granted that you get the chance to see an old friend. To paraphrase, the real tragedies in life are the ones that blindside you in an idle Tuesday. Hold those you love and have loved dear. The world lost a deeply sensitive and good person in Michelle. May her work and her huge heart be remembered by those who were lucky enough to experience her fire.”

Also, several of her Buffy the Vampire Slayer castmembers like Sarah Michelle Gellar, James Marsters, David Boreanaz, Alyson Hannigan, Charisma Carpenter, Clare Kramer and Emma Caulfield.In an Instagram post, Gellar wrote, “Michelle, listen to me. Listen. I love you. I will always love you. The hardest thing in this world, is to live in it. I will be brave. I will live… for you.”

Boreanaz wrote on his Instagram Story: “So very sad… horrible news. RIP and prayers to her and her family.” Ed Westwick of Gossip Girl shared on his Instagram Story: “So sad to hear of the passing of @michelletrachtenberg sending prayers.”

Trachtenberg’s ex-boyfriend and X-Men actor Shawn Ashmore shared a tribute, as well as actress Kate Beckinsale, who wrote on her Instagram Story: “What a terrible shock and loss, my god. My heart aches for your mama. Fly safe little bird.” Designer Christian Siriano also wrote on his Instagram Story: “My love Michelle you will be forever loved and missed by so many and I will never forget all of our memories together for the rest of my life.” Additional celebrities like Kenan Thompson, Chris Colfer and more shared heartfelt words remembering the star.