Fitness guru Richard Simmons died Saturday at the age of 76, his representative and law enforcement sources confirmed to multiple media outlets. Simmons, a best-selling author and star of millions of exercise videos - including the Sweatin' to the Oldies” line - had just celebrated his birthday on Friday. He had posted a thank you on social media for all of the messages he'd received, less than 24 hours before his death. "Thank you…I never got so many messages about my birthday in my life! I am sitting here writing emails. Have a most beautiful rest of your Friday," Simmons wrote in a social media post Friday afternoon. TMZ, which was first to report his death, said emergency crews responded to a call from a housekeeper Saturday morning and pronounced Simmons dead at the scene. According to the outlet, no foul play is suspected. Law enforcement sources confirmed to NBC News that Simmons was found unresponsive Saturday at his Hollywood Hills home. A representative for Simmons confirmed the fitness icon's death to ABC News. Simmons has been out of the public spotlight since 2014, an absence which inspired the hit 2017 podcast “Missing Richard Simmons.” Earlier this year, Simmons spoke out against a planned biopic about him starring Pauly Shore.