Turn the thermostat to fan only and insure you get air blowing through the registers or feel air pulling in at the return air. If YES then make sure you have the thermostat programmed correctly to call for cooling when set to auto! Also the start/run capacitor may be failed out side at the condensor unit. They are cheap to replace but dangerous to handle so becareful. If NO then you are not getting signal to the blower and may have a failed transformer that converts 120v to 24v for the thermostat controls. Also double check your breakers in your service panel to see if they tripped for some reason. A failing or weak breaker can cause this symptom aswell!