I have a GMP condensing gas furnace (made by Goodman Manufacturing Company, furnace model GMPN100-4 Rev A), which provides both heat & cooling to the house.

When the house gets warm, the AC comes on and everything looks good, but starting yesterday when the temperature is cooled to the set one, the compressor turns off but the fan keeps running. Seems the only way to turn it off is to turn off the power switch (on the house power panel) to the furnace. Otherwise it just keeps going nonstop. Any idea how to fix it?

Are you sure someone didn't flip the fan switch by accident on the thermostat to the on position.