By Lisa Respers France, CNN

(CNN) -- Kristoff St. John's fiancée Kseniya Mikhaleva has one question: "How did it happen?"

St. John, who played Neil Winters on the CBS daytime soap opera "The Young & the Restless," was found dead at a home in the Woodland Hills area of Los Angeles on Sunday afternoon. He was 52.

No cause of death was released and an autopsy was scheduled for Monday.

St. John and Mikhaleva got engaged in September and on Monday the Russian-born model posted her grief on Instagram.

"How did it happen ??? How ??? Why did you leave so early ???? and left me alone," the caption read. "I can't believe."

The caption also said "You were everything to me .... you were a loving father, a loving man,.....how ??"

The post was later deleted, but not before it was shared on social media.

She also posted Instagram stories including one showing the couple kissing and the caption "Why...just tell me why" with a broken heart emoji.

St. John joined the cast of "The Young & the Restless" in 1991 and some of his costars also mourned his loss publicly.

"Criminal Minds" star Shemar Moore, who played Malcolm Winters, the half-brother of St. John's "Young & the Restless" character from 1994 to 2005, posted a scene of him and St. John embracing.

"RIP....... This ain't goodbye Brother...," the caption read. "Watch me make you proud!! We will be together again... I LOVE YOU Kristoff!!! Just go chill out and watch me carry YOUR torch."

Eric Braeden stars as Victor Newman on the soap and shared some of his memories of St. John on Twitter Monday.

Braeden also thanked fans for their condolences.

"Sitting by myself in my car! It's raining outside, with dark clouds and wind outside... am reading your tweets about my friend KRISTOFF!," Braeden tweeted. "Thank you all for your kind words! I have none right now! He was a hell of an actor and simply one of the nicest guys ever! Can't believe it!"

"Y & R" alumna Eileen Davidson, who played Ashley Abbott, posted a portrait of St. John on Instagram.

"I'll miss you 'Steel,'" she wrote.

Several fellow celebs, including actress Viola Davis, also reacted to his death.

"The pain of grief and mental health should be addressed in the same realm as 'physical' ailments. No real words except....I hope you found peace. God bless those you loved who were left behind. With all my heart and empathy," Davis tweeted.

Fans also expressed their sorrow.

TV writer Ira Madison III shared a common sentiment among African-American fans who felt that the Emmy and NAACP award-winning St. John and his role on the "stories" was an important one in their households.

"The first representation on TV I remember is seeing The Winters family watching Y&R with my grandmother and extended family," Madison tweeted. "It's why I still watch soaps to this day. Thank you for being a part of that, Kristoff St. John."