Fitness guru Richard Simmons has died, two law enforcement sources confirmed to NBC News.

Simmons was found unresponsive Saturday at his Hollywood Hills home, one day after his 76th birthday, the law enforcement sources said. The exact cause of his death is pending.

On Friday, Simmons shared a message on social media thanking fans for the birthday wishes.

“Thank you…I never got so many messages about my birthday in my life!” Simmons wrote. “I am sitting here writing emails. Have a most beautiful rest of your Friday.”

He signed the post “Love, Richard.”

Simmons, the fitness coach known for his eccentric personality and “Sweatin’ to the Oldies” workout videos, rose to fame in the 1970s and 1980s after opening a number of gyms and releasing dozens of fitness videos.

His website describes him as “one of the world’s most revered and iconic fitness personalities” who has been an “instructor and motivator for over 40 years.”

“By delivering a serious message with his trademark humor, he has helped millions of overweight men and women lose more than 3,000,000 pounds by adopting sensible, balanced eating programs and exercise regimes that are energetic, fun and motivating,” a biography for Simmons on his website reads.

Simmons grew up in New Orleans and weighed 268 pounds when he graduated high school, according to his website, which added that he “took control of his weight” by changing his lifestyle to include moderate eating and exercise.

From there, he took his talents to the West coast, moving to Los Angeles in 1973, where he was unable to find a gym that “wasn’t for people who were already in shape,” according to his website. So, Simmons took it upon himself to create one, and SLIMMONS in Beverly Hills was opened in 1974.

It was at SLIMMONS where he taught classes and hosted seminars until 2013, his website said. His success also led to his own nationally syndicated series, “The Richard Simmons Show,” which ran for four years and earned multiple Emmy awards.

Simmons also released 65 fitness videos over the course of his life, which sold over 20 million copies, and authored nine books and three cookbooks, according to his website.

Simmons had been something of a recluse since 2014, and in January, he spoke out against an upcoming biopic being made about his life starring actor and comedian Pauly Shore, which Simmons said he never permitted. 

“I have never given my permission for his movie. So don’t believe everything you read,” he wrote on Facebook at the time. “I no longer have a manager, and I no longer have a publicist. I just try to live a quiet life and be peaceful. Thank you for all your love and support.”

Since that post, Simmons has been active on social media, often writing motivational messages and sharing stories about moments and people in his life. The posts have ranged in topic from his childhood in New Orleans to racial segregation to fitness and his family.

His death comes just months after he posted a cryptic social media message, writing “I am ….dying,.” He later walked it back, but the next day, he shared that he had been diagnosed with skin cancer.

In the March 18 social media post, he wrote that he had “news” to share.

“Please don’t be sad. I am ….dying. Oh I can see your faces now. The truth is we all are dying. Every day we live we are getting closer to our death,” Simmons wrote.

“Why am I telling you this? Because I want you to enjoy your life to the fullest every single day,” he continued in the post. “Get up in the morning and look at the sky… count your blessings and enjoy.”

The rest of the post included suggestions on how to lead a healthy lifestyle and reminders to hug the ones you love.

Later that day, he posted another update clarifying he was not dying.

“Sorry many of you have gotten upset about my message today. Even the press has gotten in touch with me. I am not dying,” he wrote. “It was a message about saying how we should embrace every day that we have.”

The next day, he shared that he had been diagnosed with Basal Cell Carcinoma, a type of skin cancer.

The day after that, he shared in another social media post that the issue was resolved after three procedures with a “cancer doctor,” Dr. Ralph A. Massey.

It was not immediately clear if Simmons’ recent skin cancer diagnosis had anything to do with his death.