During the Primary Survey, you will determine whether or not your casualty is unresponsive but breathing (unconscious) or unresponsive and NOT breathing, in which case they are dead and you will need to undertake CPR.
It may be, though, that the casualty has simply 'fainted'. This typically happens because they have 'been standing too long', their blood pressure has dropped and they have simply passed out. The only way you'll know is if an observer describes them as having simply 'passed out'. There will appear to be no obvious reason for it, and faints usually only last for a few minutes.
In this case, the best treatment is to lie them down on their backs, raise their legs by placing their feet on something (like a cushion) and keeping a close eye on them to make sure they are breathing. It might be worth covering them, as well, to keep them warm, and, if you are in a hot, stuffy, environment, open some windows to let fresh air in.
As they come round, reassure them, and let them gradually sit up. If, on doing so, they start to feel faint again, ask them to lie back down and raise their legs again. Once they are able, confirm with them whether this is a common occurence or just a one off. It may be they need to get their blood pressure checked by their GP.
If, after lying in the position described for 5 minutes they haven't come round, place the casualty in the recovery position, continue to monitor the casualty, and seek medical assistance.