The cause might be anything from physical destruction or malformation to environmental changes (absence of food, extreme changes in the environment, radiation, etc.)
But once the autonomous single celled organism is no longer communicating - it "achieves room temperature" (2nd law) just like the dead guy. It succumbs to entropy.
But no living single celled organism has ever done that. Each and ever one that is now alive has been alive for as long as there has been life.