I think for me Atlantic City has to be one of the top candidates for this question. It was the "place to be" among the upper class in the 1920s and continued to be a popular tourist destination until the 1950s.
However, in the decades since it has declined into a shell of its former self. With the rise of Las Vegas' now famous casino strip, newer modern casinos being built in neighboring states, and the democratization of long haul air travel, Atlantic City essentially got left behind. Nowadays, it's a place plagued with urban decay as many former casinos have closed and sit empty while the area near the once glamorous boardwalk has become really sketchy.
Which other places like this come to mind?