He was playing hockey with his son when he experienced chest pain and then vomited. He was rushed to the hospital where he passed.
From the Los Angeles Times:
The Times spoke with Robin Thicke on Tuesday evening, and he called his father “the greatest man I ever met.” He confirmed that Thicke suffered a heart attack while playing hockey with his son Carter, and that one of the last things he said was a compliment to his son on a nice shot.
“The good thing was that he was beloved and he had closure,” said Robin Thicke, who credited his dad, an accomplished musician himself, with being an inspiration and very supportive of his music career. “I saw him a few days ago and told him how much I loved and respected him.”