Unconsciousness differs from simple sleep
in that an unconscious person cannot be woken even with strong stimuli
like pain (a good way to test this is by pinching them strongly in the side of the armpit).
True unconsciousness is a severely and immediately life-threatening condition!
This is because it can disable several important reflexes related to breathing, namely the reflex that keeps your tongue from slipping back and sealing your windpipe, and the coughing reflex which prevents you from suffocating on your own vomit (aspiration). And unfortunately, many causes of unconsciousness also induce vomiting...
The best way to prevent this is by immediately placing unconscious persons into a stable side position (also known as lateral position or coma position); this is one of the most important lessons ins first aid course.
The stable side position has definite priority over
the often-heard principle to not move injured persons to avoid making the injury worse including suspected spinal injuries! It's better to be paraplegic than to be dead. Of course, you should still be as careful as possible.