Michelle Trachtenberg, 'Gossip Girl,' 'Buffy the Vampire Slayer' Star, Dead at 39

10:55 AM PT -- Law enforcement sources tell TMZ ... Michelle Trachtenberg’s body may have rejected the liver transplant she received within the last year, causing complications. We're told Michelle's mother found her.

Michelle Trachtenberg, known for her roles in "Gossip Girl" and "Buffy the Vampire Slayer," has died.

Police sources confirmed the shocking news to TMZ, though the exact cause of her death remains unclear. We're told police and paramedics responded to a call for a woman in cardiac arrest, and found Michelle unresponsive. She was pronounced dead on the scene.

A separate source tells us the actress had received a liver transplant recently ... meaning within the last year. At this point, police reportedly believe she died of natural causes.

We're told Michelle was out with friends Thursday night of last week at Sartiano's in NYC.

In recent months, she had posted photos where fans felt she appeared gaunt and frail.

Michelle defended her appearance, calling out "haters" and saying, "Explain to me how I look sick." She also reminded followers, "Did you lose a calendar and not realize I’m not 14. I’m 38. How sad for you to leave such a comment.”

In another post, she made it clear she never had plastic surgery, and was "happy and healthy."

Michelle rose to fame as a child star in "Harriet the Spy" -- later becoming a household name in her role as Dawn Summers on "Buffy the Vampire Slayer," then took on the Upper East Side as Georgina Sparks in "Gossip Girl."

A rep for Michelle tells TMZ ... “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 NY Post was first to report her death.

Michelle was 39.