How to fix Hotpoint RF452-not cooling but light on?

There is no cooling in fridge or freezer. Electrics on light is on and strained hum from back (like electric motor trying to run), not the normal noise of it working. Thermostat does not click, nor does extra switch for fast freezer cooling appear to have any effect. If the thermostat is at fault why doesn't freezer work? Also is it possible to test fridge freezer with thermostat removed before ordering one?
Thank you for your help regards Steve