If readers are lucky, the author had enough warning or foresight to keep notes on their plans for the rest of the series or at least fill someone else in on the most relevant plot points, so that their work can be continued by another author, or possibly even several.
This may result in the series Jumping the Shark if the replacement isn't very good, but at least you have closure.
Please note that this trope applies when a creator (writer, director, etc.) ceases work on a project, not a participant (actor, well... Also note that since all creators are mortal, this is not merely when creators die, it's when they die with works still in progress.
This might result in The Character Died with Him, where the show goes on without the character a dead actor portrayed, or Fake Shemp, where there is an attempt to disguise the absent actor.
In other words, “What are the molecules that allow bacteria to find and destroy viral DNA?