There are two types of unconsciousness: stunned and negative health. Stunning can occur in many situations, such as: having a toolbox hit your crotch and getting gassed with N2O.
Negative Health, on the other hand, can occurdue to receiving 100% or more damage, which places You into Dying state. During Dying state You cannot breath, and will slowly take suffocation damage until you reach -100 health, which causes death.
When you are unconscious there is nothing you can do but wait.
If you're in range of people talking it will say "You Can Almost Hear someone Talking....."