I thought I'd be a ray of sunshine and talk about death in sitcoms, not "Henry Blake" type deaths but ones where the actor's unexpected death actually takes the wind out of a series.
The first one I can recall is actor Jack Soo who played Detective Nick Yemana on "Barney Miller" for 80 episodes before falling to Cancer in 1978. "Miller" had a truly great ensemble cast and Soo was a big part of that, his laid back style and incredibly wry wit were sorely missed. Without a doubt, Yemana was my favourite character on the series, watch the clips, the humour is actually timeless.
Perhaps because of the great ensemble and good writing, "Barney Miller" would run another 80 episodes and remain a quality program, this is one of the few examples of a show that survived a sudden cast member death as we'll explore the rest of the week.