According to Kitty Kelley's biography of Sinatra, it's rumored that Lee Harvey Oswald watched this film just a few days before assassinating President John F. Kennedy, a fact that Sinatra learned years after the tragedy, prompting him to withdraw the film from circulation. After that withdrawal, there was a failure to renew the copyright and, ironically, it fell into the public domain. As a result the film became widely available from a number of discount/public domain distributors. The film also became part of the colorization controversy in the mid-1980s when Suddenly was colorized for home video—turning Sinatra's blue eyes brown.
You can watch the entire movie at the Internet Archive. Suddenly is considered to be part of the public domain.