The pilot was making a night visual approach to the end of a strip which was to serve as a helipad. The pad was illuminated by the headlights of a vehicle while lights from a house and shed provided additional peripheral light sources. The pilot reported that there was considerable turbulence adjacent to high ground near the strip. The approach was being flown at an indicated airspeed of 50 knots. While passing over the high ground at a height of about 400 feet, the airspeed suddenly reduced to virtually zero and a large sink rate developed. The pilot attempted to go-around but the helicopter continued to descend until it struck the ground in a near vertical descent. The terrain and wind velocity were such that windshear over the high ground was probable. However, it was considered unlikely that this alone would have resulted in such a severe loss of airspeed. It was possible that the pilot, who had limited night flying experience, had not noticed a decay in airspeed prior to the sudden descent. If this was the case, the conditions were suitable for the aircraft to enter a vortex ring state. The fact that when the pilot increased power and collective the descent continued would also support this conclusion.