Why does anoxia cause cell death?

If the only issue were the ability of the cell to consume nutrients and produce the energy necessary to survive, then your "hibernation" theory would work.

The major problem is that the molecules inside the cell (DNA, RNA, proteins) are spontaneously decaying with time and the cell is constantly replacing them. In the absence of oxygen the cell cannot create the energy necessary to balance the decay and the cell does not have enough machinery to "restart" when oxygen returns.

The details can get more complicated when you start in on proteases and nucleases and whatnot, but that is the basic version of why.

To follow on Tadgie's answer:

1) Ca-dependent kinases and proteases certainly play a major role in cell death, but the Ca influx alone does not cause this. Most of these processes, inlcuding the kinases by definition, require the same ATP that no longer exists to do their jobs.

2) This pehnomenon has nothing to do with calcificatiosn in teh elderly.

mdgcats is not exactly right. Hemoglobin does not have anything to do with nutrients. Hemoglobin, as a transport protein, simply transports the oxygen. However, anoxia causes cell death since the only metabolic pathway the cell can undertake is glycolysis, which is a very energy inefficient pathway. And gene regulation would be involved in changing the activities of the cell and as you said there are many important required processes that must take place and can't be negatively regulated without killing the cell.

The primary goal of respiration is to generate ATP, which is the main energy source for most of the reactions that go on in the cell. Without the ATP, the normal functions of the cell that keep it at balance fail. The main and most damaging function is that of calcium pumps in the membrane of the cell. Normally, the cell pumps ions like calcium out for a number of reasons, one being that calcium activates a number of enzymes that shouldn't be activated, which break down proteins and DNA in the cell. The pumps require ATP to pump the calcium out of the cell, and without oxygen, the cell cannot generate enough ATP to keep the calcium out. There are other things that occur in cell damage and death, but calcium influx is one of the most obvious and damaging.

That's the basics as to why cells die due to anoxia. Because of this, calcium is usually a pretty good indicator of cell injury and death. This is also why many parts of elderly people become hard, the persistent insults over time kill cells, and they become calcified and hard.

If you don't give it energy, it cannot do anything and after a time of not doing anything, a cell loses its ability to do anything. Thats a death of a cell. Im not sure specifically what causes it on a molecular level though

Cells need oxygen to live. The oxygen is carried in the blood as hemoglobin. Without proper nutrients, there is too little hemoglobin to support the body's oxygen needs.