My thermostat's display is showing - AC and the heat will not come on, I removed it and replaced the batteries to no avail? What can I do to get the heat to work again? It is 40 degree's here

The thermostat is missing the 24 volt circuit from the furnace. The exact meaning is no power to the thermostat according to Honeywell.

Could be a 24 volt transformer/fuse/triper breaker in the electric panel/etc.