Someone unresponsive

The most time-critical case.

Assess

Shout and gently shake the shoulders. If there is no response, open the airway by tilting the head and lifting the chin, and look and listen for normal breathing for up to ten seconds.

Occasional gasping is not normal breathing; treat it as cardiac arrest.
If breathing normally, place in the recovery position and monitor.

If not breathing

Call emergency services, ask for a defibrillator, and start chest compressions immediately. Every minute without them reduces survival substantially.