My fan is not turning what could this be ?

My fan is not turning what could this be ?

It could be the lack of a low voltage signal from the thermostat, low voltage transformer. It could also be the contactor in the outside unit, It could be a defective fan motor or a defective fan motor capacitor. In other words it could be one of many items so you need to test the unit to find the defective part. Just buying parts and guessing if they are defective gets very expensive and cost prohibitive. Sorry but I cannot be there to test it for you. You might be able to test the fuses and make sure the circuit breaker is on.