I do have an old C500EC40E wiring diagram which shows an optional fault detector reset switch. When a fault is detected the switch trips and opens the circuit to the key switch. This is likely what that switch is. An option
would be to return the circuit to original.
As far as diagnosing faults go, first start with the battery, make sure is fully charged and no corrosion. Controllers can fault out from corroded or dirty batteries due to negative voltage.