Heater fan is turning on, Fluid is full just changed thermostat. What else? on 2000 Lincoln LS

I bought the car in the winter of 2010 and everything ran fine. Now it is getting cold again the heat doesn't work anymore. I know for sure it isn't the heater core, Thermostat or fluid. Everything seems to be in shape but can't find the problem. I'm not very mechanically inclined so as much help as possible would be very appreciated. Thank you

Asked by for the 2000 Lincoln LS