the on/off switch on my qualcast lawn mower won't work, what tools do i nee?

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

Ideally a screwdriver, spare switch and COMPETENCE TO WORK ON AN ELECTRICAL ITEM.

The simple fact that you have no idea from where to get tools (hint: Homebase, B&Q or any DIY shop) or spare parts (hint: G00gle search) suggests it would be extremely dangerous to use something you have repaired

Dkmdj
March 2020