My thermostat is stuck on AC, but the AC or heat are both NOT working

Do you mean "AC" appears on the display? If so, this may indicate a loss of power to the system, particularly if your thermostat is a Honeywell Chronotherm. Try moving fan selector switch to "on". Does the furnace blower run? If not, check power supply/fuses/circuit breakers. If you have a gas furnace, check that the blower access panel has not come loose and disengaged the safety interlock switch behind it.