The mishap occurred during a practice hover with a 100 ft cable in preparation for an external load operation. While hovering about 100 ft AGL, the fuel boost pump light illuminated and the boost pump pressure dropped to zero. A descent was started; however, while still at about 100 ft AGL, the engine lost all power. The pilot entered an autorotation, but the aircraft landed hard. The pilot noted frost on the fuel cap that was not present during the preflight inspection. Also, a small amount of ice was observed floating in the fuel about 45 minutes after the accident. During a subsequent examination of the engine and aircraft, water was found in the fuel tank, airframe fuel filter, engine fuel pump filter and high pressure fuel filter. The fuel supply line from the high pressure fuel filter to the fuel control was empty. Anti-ice fuel additive was not required in the fuel, since this Bell 206B was equipped with an airframe fuel filter. The source of water/ice could not be determined. Temperature minus 1 deg F.