Unconsciousness is a strong indication of a serious medical situation and complicates a number of injuries and illnesses. Breathing difficulties can easily arise as a result of unconsciousness; the airway may have become blocked by the tongue or fluids. The first priority upon finding an unresponsive casualty is to ensure a clear airway is maintained.

Remember:

"Look-for chest movement Listen-for sounds of breathing Feel-for breath on your cheek" (pg. 2-9 St.John's Ambulance First Aid Guide)
Recheck breathing frequently
If at any point the casualty stops breathing perform artificial respiration immediately

Procedure

send for medical help
perform a primary survey
perform first aid for any life threatening conditions
loosen clothing
if there are no spinal/head injuries, place casualty in the recovery position

Fainting is when there is a brief shortage of oxygen to the brain causing a temporary loss of consciousness. A number of problems such as hunger, fatigue, lack of fresh air or long periods of sitting/standing can cause it.

Signs

Before the a person faints they may exhibit the following symptoms:

paleness
feels sick or dizzy

Procedure

lay the person down with legs raised or have them sit with their head lowered between their legs
make sure the casualty has access to plenty of fresh air
loosen clothing

If the person has fainted:

follow the same steps as for an unconscious casualty
when the casualty regains consciousness, keep them lying down for 10-15 minutes