Ritter became ill Thursday while working on his ABC series and underwent surgery at Providence St. Joseph Medical Center in Burbank for a tear in his aorta, a rare medical condition that can hit without warning.
As CBS' Elizabeth Kaledin reports, the aortic dissection – a tear in the aorta -- that felled Ritter is tough to detect and leaves little time to react. It affects 1 out of 5,000 and few survive it.
He died shortly after 10 p.m. Thursday, publicist Lisa Kasteler said. He was accompanied by producers, co-workers, his wife and his 23-year-old son, Jason, said Susan Wilcox, his assistant of 22 years.
Ritter's youngest child, Stella, turned 5 the day he died. His 55th birthday was next Wednesday.
The son of Tex Ritter, a Western film star and country musician, Ritter was an effortless funnyman who — given the chance — could handle drama as well. Friends recalled him as loving and buoyant.
"It's like there is a big tear in the world's heart," actor Henry Winkler told "Entertainment Tonight" on Friday. "He was extraordinary in every aspect of his life, especially as a father. His children were there at every moment of his life."
Winkler, who co-starred with Ritter on Broadway in Neil Simon's "The Dinner Party," was to make a guest appearance on the ABC sitcom. He was on the set Thursday for rehearsal when he was told Ritter had taken ill.
No decision had been made Friday about the future of "8 Simple Rules...," which was to begin its second season Sept. 23, an ABC spokesman said. It's one of the few bright spots in the struggling network's lineup. Three new episodes had already been filmed, and Ritter was working on the fourth when he fell ill.
"He was more than a comic," Simon said in a statement. "He was a real actor with a genius for comedy. I loved his performance in 'The Dinner Party."'
"I'm shocked and heartbroken and so sad for his family. I cannot find words to express my sorrow — such a great loss to the joy in the world," Joyce DeWitt, who co-starred with Ritter and Suzanne Somers in ABC's "Three's Company," told "Entertainment Tonight."
The sitcom, which aired from 1977-84 and brought a new level of risque humor to TV, was the No. 1 comedy in the 1979-80 season and regularly part of the top 10.
Ritter played a handsome but goofy bachelor who hinted he was gay so he could live with his two female roommates without raising eyebrows. Sexual double-entendres were the order of the day.
Behind the scenes, Somers' money demands led to clashes with Ritter and DeWitt, and she was eventually written off the show. In a tearful interview Friday on Fox's "Good Day Live," Somers she had reconciled with Ritter at the request of his wife, actress Amy Yasbeck.
She and Ritter had "this phenomenal phone call" in which they worked out all their old differences and had planned to work together again, Somers said. "I learned so much from him. ... He was the best physical comic I've ever watched."
"It's just stunning, unbelievable," said assistant Wilcox. "Everybody loved John Ritter. Everybody loved working with him. ... Whatever set he was working on, he made it a very fun place."
ABC had a rare recent success with "8 Simple Rules" and used it as the lead-off series of its Tuesday lineup. Its second season, which was to begin Sept. 23, was postponed Friday and production was halted.
"All of us at ABC, Touchstone Television and The Walt Disney Co. are shocked and heartbroken at the terrible news of John's passing," a statement read. "Our thoughts and prayers are with his wife and children at this very difficult time."
Ritter, a Southern California native who lived in Beverly Hills, had appeared in more than 25 television movies and a number of films.
The youngest son of Tex Ritter and actress Dorothy Fay, he graduated from Hollywood High School and earned a degree in drama from the University of Southern California.
"I was the class clown, but I was also student body president in high school," he told The Associated Press in a 1992 interview. "I had my serious side — I idolized Bobby Kennedy, he was my role model. But so was Jerry Lewis."
He received an Emmy, Golden Globe and other awards for his "Three's Company" role and was honored by the Los Angeles Music Center in June with a lifetime achievement award.
Ritter appeared in more than 50 plays nationwide and won critical acclaim for his recent nine-month run in Simon's production. He had a memorable turn in Billy Bob Thornton's 1996 film "Sling Blade."
"I think people recognized that although his success was in sitcoms, he had a real talent for acting," said AP contributing writer Bridget Byrne, who knew Ritter for 30 years and recently visited him on the "8 Simple Rules" set.
"He talked a lot about his real and TV families, and seemed in a good place with both," Byrne said. "He was enjoying what he felt was the perfect life."
Ritter's first steady job was his role as the minister in "The Waltons." He worried about falling into a typecasting trap after "Three's Company" ended.
"I would get scripts about 'a young swinging bachelor on the make,' and I said 'No, I've done that,"' he told the AP. "What I was looking for in my time off was something a little bit different, a little serious, or funny in a different way."
Ritter described his time on "Three's Company" as "an education" in quick-study acting.
"When the curtain went up, no matter how long you've studied or haven't studied at all, you had to answer to the audience. We didn't do retakes. If there was a (microphone) boom in the shot, so be it," he said.
Ritter later starred in the TV series "Hooperman" and the early 1990s political comedy "Hearts Afire." He received multiple Emmy nominations for his PBS role as the voice of "Clifford the Big Red Dog" on the animated series.
His TV movie appearances included "Unnatural Causes," Stephen King's "It" and "Chance of a Lifetime."
Ritter won popularity among independent film directors in recent years and, besides "Sling Blade," appeared in "Tadpole" in 2002 and the new feature "Manhood." He appears alongside Thornton in Miramax's scheduled November release of "Bad Santa."
Ritter was married from 1977 to 1996 to Nancy Morgan, the mother of his three oldest children. He married Yasbeck in 1999. In addition to son Jason and daughter Stella, he is survived by two other children: Carly and Tyler.
Funeral plans were pending.