zanussi fridgefreezer suddenly stoppped cooling n freezing?

Everything appears to be working as in the noise from the motor sounds the same, the light in the fridge comes on but for some reason it now will not freeze or cool so i turned it off for 24 hours & put it back on & left it for 24hrs & it had still not started to freeze up or cool in the fridge area???? been told its more thaan likely the gas, then the thermostat, then the compresser then something else this is friends telling me this not an engineer, to afraid to call 1 out as i feel i am going to get ripped off.