A couple of days ago my thermostat was set to 69, but the heat didn't turn off. When I got home it was 75 in the house, so I shut off the system by turning the thermostat to off. I have tried on several occasions to get the system going again, but it's not working. I checked the breaker and replaced the thermostat and that didn't help.

The system is a 1992 York Stellar Plus. Any thoughts?

Post the actual model # of the indoor unit.