Have you tried firstly. REMOVE SPARK PLUG FROM ENGINE, and put the plug in your pocket so the engine can not accidentally fire up.
check that the piston is not seized in the bore by SLOWLY moving the blade. Use a thick glove to avoid damaging yourself.
If the engine is free then suspect the pull cord mecanism.
ALWAYS REMOVE THE SPARK PLUG WHEN WORKING ON GAEDEN MACHINERY.
Roger Roberts, April 2007