Fridge freezer failed completely

My Hotpoint fridge freezer has stopped working. Both the freezer section and fridge section have no cooling effect atall. The problem started when I noticed that food in the fridge section was freezing. However the freezer section was functioning normally. I turned down the fridge temperature control but it was still freezing food. After about a week the freezer section defrosted all of my frozen food and would not work. I tried switching the thing off and leaving it for 48 hours and than switching back on. There was still no freezing in the freezer and no refridgeration in the fridge. It isnt a power issue as the fridge light comes on as normal.
I removed the panel in the freezer section. I disconnected the thermostat and tested it for continuity. It has continuity on both the blue and brown leads.
I also noticed that when I switch the fridge freezer on the fan starts running and then switches off after about 5 seconds. Is there a clue there?
Does anyone have any idea what the fault may be. If so , I can decide whether or not it is cost effective to have it repaired.