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What process takes place when oxygen is not available for cellular respiration?

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When Oxygen is not available, the cell is forced to produce energy (=ATP) through ANAEROBIC processes, that produce much less energy (about 15 times less), than AEROBIC processes.
 
Oxygen is essential for the functioning of the electron transport chain.
The electron transport chain (ETC) allows the cell to produce energy (ATP) by creating a proton gradient across the mitochondrial membrane. 
 
The necessary steps to produce energy from glucose are:
Glycolisis --> Kreb's cycle --> ETC
 
During glycolisis and Kreb's cycle NAD+ is transformed in NADH. In order for this processes to continue NAD+ needs to be replaced, and the ETC is, in the presence of oxygen, able to do that.
 
If ETC doesn't work NAD+ is not restored and the cell needs to find another source of it.
 
In this case pyruvate, instead of entering in the Kreb's cycle, will be transformed in Lactate through the enzyme lactate dehydrogenase, and NAD+ will be produced:
 
Pyruvate + NADH ------> Lactate + NAD+
 
That why with hypoxia cells/tissue accumulate lactate, causing acidosis.
 
 
I hope this helped. Let me know if I answered your question.
Hi Raquel,
 
Respiration, cellular respiration can occur without oxygen. It is known as anaerobic respiration and uses respiratory electron transport chain. This chain performs its function through exogenous electron acceptor that allows the electron to pass through the system and must be present. In aerobic organisms oxygen is the final electron acceptor. In anaerobic organisms, Sulfate, Nitrate or Fumarate are used, these are relatively less efficient and have smaller reduction potential than O2;  Oxygen indeed is highly oxidizing agent and releases lot of energy. Sulfate and Nitrate release less energy per oxidized molecule. Therefore, anaerobic respiration is relatively less efficient, occurs in prokaryotic organisms that live in environment without O2.
 
I hope this helps, if you have questions about this  then please let me know.