The aircraft collided with a mountain during a night cross-country. The weather was VMC at takeoff but became IMC later near the mountains. A VFR flight plan was filed but never opened. The pilot continued flight into an area of known IMC conditions. He had returned to Las Vegas once and asked for more weather briefing. Shortly after his second departure, at night, he asked for ATC help for separation through Las Vegas, CA. He was observed climbing in heavy rain. About 15 minutes later at 1226, he reported navigational problems. ATC gave him a heading for a return. ATC personnel observed that the aircraft commenced a turn which tightened and then the target disappeared from the radar scope. The aircraft impacted about 65 ft below a ridge line at an elevation of about 3500 ft MSL. It impacted rising terrain with the right wing first, the aircraft traveled 58 ft upslope before coming to rest. Mountain tops in the area were obscured.