Hotpoint Undercounter fridge doesnt cool!!!!?

I have a Hotpoint Ice Diamond Fridge which has an icebox and alongside to the iceboz the temperature dial with a green defrost button in the centre. The problem is that no matter what i do the fridge doesnt turn on when i turn the temp dial, unless i hold the green button partially in! I did a check to see whether everything worked ok by duck taping the green button on but even on the lowest setting it was freezing eveything, which showed the fridge works/ cools. Anyone know what the problem could be? Any help would be greatly appreciated