Stores of glycogen in the liver can normally provide enough glucose for around 12hours but when it runs out; stored fats are broken down to release fatty acids and glycerol to become the next source of energy for the body by the process of Ketogenesis and Ketolysis:

Ketogenesis:
In the liver, the released fatty acids are converted into 3 Ketone bodies
o    Acetone

o    Acetoacetone

o    3-hydroxbutyrate ( 3-OH butyrate)

Ketone bodies are transported from the liver to other tissues such as muscles and the brain

Ketolysis

Two of the ketones (Acetoacetate and 3-hydroxybutyrate) are reconverted back to acetyl-CoA which can then be used to produce energy, via the tricarboxylic acid cycle (TCA cycle).

As levels of glucose continue to drop, the process of gluconeogenesis is also stimulated to produce energy. Oxaloacetate (OAA) is preferentially used in the gluconeogenesis process becoming unavailable for the TCA cycle. 

The TCA cycle stops but ketone bodies continue to be produced, increasing levels in the blood. Excess ketones are excreted in the urine (ketouria) or through the lungs creating a fruity smelling breath.  The ketones are acidic and if levels continue to  increase, the pH of the blood drops developing the ketoacidosis, a dangerous condition, which can change the normal functioning of internal organs such as the liver and kidneys.