cooling fan not working. on 2000 BMW 323Ci

Cooling fan isn't working. Car is not overheating I think because it's not warm enough outside. The problem started back in October and I noticed the car getting too warm and so I turned on the heater which brought the temperature back down to normal. Why would the fan stop working and what will it cost to fix it.

Asked by for the 2000 BMW 323Ci
the cooling fan will only come on when the engine gets hot if its not getting warm check your t stat