Richard Simmons -- DISAPPEARED From Sight ... Injury and Depression

2:40 P.M. PST -- Richard just posted a message on his Facebook page:

Something is seriously wrong with Richard Simmons ... he disappeared for almost the entire year ... and we're told there's a physical problem that has led to debilitating depression.

Simmons was last photographed in public January 11th at a charity event. TMZ got video of Richard later that evening, but that's the last time anyone has seen him in public.

Various people who are extremely close to Richard tell TMZ ... he has gone radio silent with them, he does not accept or return phone calls, and he has his housekeeper turn people away when they come to visit him.

Richard has not shown up at his famed studio, Slimmons in Beverly Hills, for months. Simmons was a fixture there, regularly teaching classes. The staff offers no explanation for his prolonged absence.

A source extremely close to Richard -- one of the few people with whom he is in contact -- tells TMZ the fitness guru is in a massive state of depression over a bum knee. He says doctors told Richard he needs a left knee replacement and if he doesn't get one he will never be able to exercise again.

The source says Richard is terrified of the surgery and has put it off all year. He had a right knee replacement 4 years ago and he put off that surgery for nearly a decade ... he was that frightened.

We're told Richard has tried stem cell and red blood platelet therapy -- like Kobe Bryant -- but it failed.

We're told he's paralyzed with fear and depression, which has made him a shut in for 2014.

Other friends and business associates tell TMZ they don't believe this is just a knee problem ... they think it's more serious.

We left a message for Richard, but he did not return our calls.