start it up and throttle it lightly,
check to see if the pinions (smaller gears) are turning. If no, then likely the clutch.
If YES then check pinions(larger gears),
If the are not turning, then check if either the spurs or pinions are stripped.
If spurs and pinions are good then check your driveline, But if the wheels turn eachother, then I would suspect, as alvinl says, a stripped pinion, or some problem with the clutch.