My thermostat's LCD is showing "OL". When I switch to Air Conditioning it will not do anything like it's dead. I turn off my breaker for the airconditioning/heater unit and try to power it again it will not do so. Can you tell me what "OL" means? It's a Goodman 3 ton with electric furnace in the basement

Have you tried researching the manufacturer's website? Many time if you just Google the make and model you can find pdf's for the Owner's Manual etc.