I'm just checking, and excuse me if you think i'm stating the obvious, but when you press the start button, you are making sure that the safety trigger on the side of the button housing is being fully pulled back and held in place before you press the button?
Also check the fuse in the plug is ok and that any RCD device that you are using is reset in case it has been tripped? Try plugging the mower into a completely different socket without an extension lead to see if that helps identify a problem. As previously stated, I'm also worried that your question rings alarm bells as to your ability to perform a safe repair.
Rupert, March 2020