First be sure nobody hit the switch on the thermostat that turns the fan on (instead of just being on auto). Then make sure the thermostat isn't the problem. Some are mounted with a base plate that the thermostat clicks into. If that's how your's is setup you could pop it out to see if that changes the situation. Otherwise it's likely to be control board in the air handler. Further analysis isn't possible unless you post the make and model number of the equipment.