Verona range not heating.

Verona range basically identical to a Bartezzoni. Oven itself is electric and has a thermostat and a large mechanical selector switch. After tampering with it the oven selector it started to work couple days later costumer said it failed to start. Any ideas on how these units operate? Seemed like a basic system and nothing burned or broken but seems to be failing.. thermostat? The large oven started to work after securing the leads on the selector switch.