mlnp 450hp 40?

If we assume that the pull cord mechanism has not failed, which would require a new part to sort the problem, you should first check that you are fully pulling the safety lock bar on the handlebar at the same time as trying to start the engine. Check the cable that runs from the safety bar is actually retracting the lever that it runs to on the engine.
The next thing to check is the blade will rotate by slowly turning it by hand whilst pulling the safety bar back at the same time. If you find you can't manually turn the blade, take the spark plug out to see if that helps. Before you try and turn blade by hand, disconnect the ignition lead that runs to the spark plug so the engine can't start by accident and cause injury.

Rupert
March 2020