If the fan in your oven is no longer working altogether or is blowing cold air rather than hot air, it will mean that one of two things are malfunctioning – the fan and motor assembly or the heating element by the fan itself.
If the thermostat of your oven is turned on, but you notice that the oven is blowing in cold air, the likely culprit could be the damaged heating element surrounding the fan. Have a technician unplug the cooking appliance and eject the back cover. This will give you a clear view of both the oven and the element.