A common basic air conditioning system and gas furnace application  consists of a gas furnace located inside the building with an evaporator coil and a condensing unit located outside the building usually at ground level. The gas furnace blower is used to circulate the air over the evaporator coil where it is cooled and distributed through the building via the duct system. The gas furnace has a transformer inside the control panel that reduces the 120 volt power supplied to the furnace to 24 volts used for the control power. The 24 volts is sent to the thermostat where it is switched to turn equipment on and off as required by the temperature inside the building. On a call for cooling the thermostat has 2 circuits that are energized. It will supply control voltage back to some type of device that turns on the blower motor. It also supplies control voltage to the outdoor unit to energize it. These are two totally separate circuits so that gives you a clue during troubleshooting. If the indoor fan and the outdoor unit are both off you should suspect either a loss of 24 volts from the transformer or a bad thermostat or thermostat wire. If the outdoor unit is operating and the indoor fan is not you will need to troubleshoot the fan circuit. If the indoor fan is operating and the outdoor unit is not then most likely the issue is in the outdoor unit.