1) Engine running but had no power to accelerate forward.
Somethings wrong with the clutch
2) Engine stopped running and it rolls forward without using the clutch
Somethings wrong with the clutch or ya lost compression.
Loss of compression can be a few things
1) Spark plug is loose or popped out
2) Cracked head, blown head gasket, loose head bolts,
3) Broken piston rings, Piston ring end gap stuck closed or hole in piston
4) Gouges or deep scratches in cyl wall caused by a broken piston pin clip
This would probably destroy the piston and cyl wall