Just as a multi-cellular organism has billions of cells, this civilization consists of billions of humans.
In multi-cellular organisms, cells differ in two major ways. First, they are located in different places in the organism, which means that some are closer to the surface or outside environment, and others are farther away. The second way is that the cells are actually different from one another. In humans, there are about 200 recognized different kinds of cells, each serving a very specific purpose.
Just as cells differentiate, starting with the single cell of the zygote following fertilization, so do humans differentiate, starting from newborns which are (for the most part) identical, to adult members of civilization.
In organisms, cells differentiate, forming tissues, organs, and systems. These work together to ensure that the organism survives. In general, cells transport and transform energy and/or matter.
Organisms, and civilizations, die.
All organisms in the past have died. All organisms currently living will die. All past civilizations have died. This civilization will die as well.
What leads to death? Typically, the death of a group of cells leads to death. It could be cells in the heart, or cells in the brain, or blood cells. The reason for cell death is because the environment changes in some way: too much or too little of something. For heart attacks, it is due to too little oxygen. Then, the heart can stop working, and then the blood stops moving, and then the entire body runs out of oxygen, causing the death of the entire body.
Civilizations die for the same types of reason. An essential system breaks down. That system fails due to the failure of the most critical organ of that system. That organ fails due to a breakdown of part of