First make sure the large spur gear from the clutch bell isn't stripped. If it's good, try holding it still while you start to rev the engine slowly. If you can hold it back, there's a problem inside the clutch bell. If the clutch engages, and you CAN'T hold it back, the problem might be in the trans. Also, check your driveshafts underneath.
You also need to make sure that all 4 wheels are properly mounted on the hexes. If one is broken or stripped, or a pin is missing, it will spin freely under power and not transfer any power to the other 3.
Keep us posted, we'll figure it out.