Unconscious casualties or semi-conscious casualties demonstrating signs of partial obstruction caused by the tongue should have their airways opened with the head-tilt/chin-lift or the jaw-thrust maneuver.
As mentioned previously, muscles of the tongue may have relaxed, causing the tongue to occlude the airway, and using the head-tilt/chin-lift or jaw-thrust maneuver may move the tongue and open the airway, allowing the casualty to resume breathing on their own.
If you suspect that the casualty has suffered a neck or spinal injury, use the jaw-thrust method (if a First Responder is available and able to maintain the jaw-thrust maneuver).