your clutch shoes could be worn. if you have a racing clutch. the spring could of snapped. if your car has a slipper clutch that could be loose? or it could be a diff problem. as they spin freely when lifted off the ground, but go nowhere when placed on the floor due to weight. it could be a dumb question but do you have it tuned proply. if it is too rich it will be sluggish and not move but when lifted off the ground the wheels will spin as there is no weight. but check the clutch sping/shoes, your slipper clutch and diffs.