He tried to call an ambulance but wasn't able to dial the phone. He thought that it was the final moments of his life. It was when he went into another room and asked someone to call an ambulance for him. On his later blog post on January 5th, he revealed that it was a "Myocardial Infraction" also known as a heart attack. His doctor stated that 99% of the blood vessels on the right on his heart was clogged.
After he was rushed to the hospital, Hayami discussed that the pain was unbearable, but since he was once a Kamen Rider, he didn't admit that he was in pain. Instead, Hayami said, "It hurts... you!" He posted all of these information after he was well rested because he knew that the fans would be concerned and wanted to update everyone about his current condition.
[Source: Yahoo! Japan via Daisuki Toku]