Fans showed up at his Hollywood Hills home, balloons in hand, to wish the fitness guru well. Richard did not come out of the house ... which is the way it's been for years.

Richard hasn't been seen in public since 2014. He was everywhere before that, but suddenly retreated from the public eye.

TMZ produced a documentary in 2022 on Richard -- "TMZ Investigates: What Really Happened to Richard Simmons" -- and after it aired Richard actually responded to fans who were expressing their concern and sending their love.

As we said in the doc, Richard has knee problems and did not want to appear in public when he wasn't at his best. He was also emotionally drained from his nonstop concern for just about anyone who reached out to him for help.

We're told he still goes out in public, but wears disguises good enough to fool even the paparazzi.

Happy 75th, Richard!!!