The thermostat allows you to set the internal temperature of your fridge-freezer and maintains it at the right level.
Are you finding that your fridge is not getting cold enough, or the reverse is happening and it's getting to cold? The thermostat could be the cause.
It's possible that the light might come on okay but the motor won't operate, in which case the fault will probably be with the thermostat.
Replace the thermostat on your fridge-freezer by ordering a new part here. Find the right model to fit your appliance and restore it to full working order again.