The private pilot was at a cruise altitude of 12,000 feet in instrument meteorological conditions on an instrument flight rules flight plan when he experienced a total loss of engine power 6 minutes after switching fuel tanks. The pilot initiated an emergency descent and broke out of the clouds at approximately 300 feet above the ground and landed in a clearing with the landing gear retracted. The airplane slid approximately 300 feet and impacted a fence. The pilot and his two passengers were not injured. The airplane sustained substantial damage. The reason for the loss of engine power could not be determined.