He'd been fighting with lung cancer, emphysema and pneumonia. He was 72. Damn, I liked that guy. You probably best remember him as "The Riddler" on the 60's Batman TV show, but he was an extremely accomplished impressionist, one of the men that fuelled my enjoyment of doing voices and impressions of my own. Without this kind of influence, I wouldn't be the gamer/DM that I am today...and I'd have laughed a lot less.
Sleep well, Frank. You've done your time, you earned the rest.