how to fix the handle/switch of Karcher Floor polisher FP222?

I have a Karcher Floor polisher FP222 which waxes and polishes solid floors. Its on/off switch is set by the position of the handle. A small (~2cm long) piece of metal has fallen out and now the handle will not stay up. it still works, but I have to turn it on and off at the mains, as I put the handle in the 'off' position, an dthen it immediately falls down back into the 'on' position. anyone know how to fix this? I do still have the piece of metal....!