Poor circulation to the vital organs.
Dilated blood vessels, bleeding, severe dehydration, all leading to a drop in blood pressure, which results in poor circulation. These can be caused by severe emotional trauma, physical injury, illness, etc.
Unusual behavior (e.g. very calm or very anxious), lack of pain to an injury, rapid breathing, rapid but weak pulse, bluish skin (cyanosis), unconsciousness.
Activate the EMS right away. Assist the person to lie on their side to improve circulation, treat any injuries, help them take any medication for an illness.
Shock is very serious and life threatening. The casualty may not know that they are in shock. Stay calm, make sure they rest, and reassure them that help is on the way.