Organizers of the Download Festival said the rocker, 37, had been hospitalized in Nottingham, which is 99 miles north of London. He was suffering from stomach cramps and dehydration, his spokesman told the music magazine NME.
Rock had been due on the festival's main stage around 3.30 p.m. Friday, but an announcement came over the public-address system a few minutes before saying Rock was unwell.
He had reportedly attended a late-night event at Central London's Whisky Mist nightclub that had continued into the early hours of June 13.
The performer had stirred controversy in the U.K. earlier in the week after telling a black interviewer that he thought the confederate flag was "cool".
Rock also revealed he's patched up his feud with Tommy Lee after punching the Mötley Crüe drummer at the MTV Video Music Awards in 2007. "We're friends," he told The Guardian. "We don't talk all the time but everything's good."
And what about his relationship with his former wife of four months, Pamela Anderson (who is reportedly back together with Lee)? "If I see her, I say hello, but I kind of don't want to be on that scene." Rock says. "I touched the stove, it's f------ hot, I don't want to touch it any more."