Why isnt my fridge getting cold?

i have a 2.7 cu-ft black & decker mini fridge. It was doing fine at first now the small freezer compartment is over freezing, theres frost everywhere covering the whole compartment. i have the setting at 51/2( between the 5 and 6 setting). thats where i've always had with no problems now the inside doesnt seem to be getting cold enough but the freezer is to cold. i read the reviews and someone was saying wrap the thermostat bulb with the plastic that came with the fridge i did that. the plastic wont stick anymore. i dont know if the thermostat wire is supposed to be on the inside of the freezer compartment or on the outside bottom of the freezer compartment and i cant find my manual. Someone please help me!!