BEKO LA 720 undercounter fridge not cooling?

I recently got a BEKO LA 720 fridge from a friend because he needed a bigger one,it is only about three years old.When I brought it home I left it stand for two days to let it settle-before plugging it in.It worked fine-cooling & defrosting-motor cutting in & out for about three weeks -but then it stopped cooling but the motor was running all the time,there does not seem to be any damage to the cooling coils, motor housing or pipes,any ideas wou"ld be appreciated.