What causes acute kidney failure?

Here are a few of the causes:

Bacterial and viral diseases. Strep, for example, can attack and scar the kidneys.
High blood pressure (often part of the complications of diabetes).
Limited blood flow (due to heart or artery disease or severe blood loss)
Severe dehydration.
Injury.
Certain abused drugs including alcohol (but liver damage is usually present first).
Blockages including kidney stones.

The kidneys typically do not ever gain back the function that they have lost due to any of these causes. They do not heal. The body has only a very small ability to use other systems to do the kidney's functions (the skin can eliminate some toxins and excess salts, for example) so that a person in acute kidney failure will become dependent on dialysis or require a transplant if renal function falls below around 15% or so of normal.