AC in home was working and the fan is still blowing air but when the AC is turned on the fire starts up in the furnace. Can someone help me troubleshoot? I have changed the thermostat in the house and changed the filter already.

How do you have the thermostat wired?

And what type of thermostat did you install ? Did this problem exist before the thermostat change? If you put in a digital thermostat you may have to tell it that you have a gas furnace, it may be operating in the heat pump mode.