The one by the power switch? That is a basic on/off. No trick to it.
Remove the head, get a tester, recline the machine, see if there is power coming to the two terminals. If not, try the brush switch up top once or twice. If not, check the wires linked above, if they are OK, suspect the microswitch
Its simply a case of following the lack of juice back bit by bit from the cleanerhead. Or down from the switch; whichever you prefer.
Having ruled out the cleanerhead, in order of what we use and what we sell, the loom is next likely culprit followed by very occasionally the microswitch. The switch up top is least likely, they are a very good switch they have used for years across many models.