“Piano Man” fans were stunned when Long Island music treasure Billy Joel was forced to cancel his upcoming tour dates because of a shocking new medical diagnosis. The 76-year-old “Uptown Girl” singer was diagnosed with a rare brain disorder known as normal pressure hydrocephalus or NPH. The Post spoke with Dr. Charles Matouk, the director of the normal pressure hydrocephalus program at the Yale School of Medicine, to help understand what NPH is, how it’s treated – and what the outlook might be for the legendary performer.