LYM123 go back and add your own post.
First, check the circuit breakers again, Then, does the thermostat use batteries? Maybe they need replacing? Perhaps the programs in the t-stat were altered? If all this is OK, turn off the power to both the outdoor and indoor sections. Take the t-stat off the wall and make a sketch of which color wire goes to which terminal. Remove the wires from the terminals. Twist together the bare copper ends of the wires from terminals R,G and Y, making sure that you have a good connection. Set the wire from terminal W out of the way. Turn on the indoor section first and then the outdoor section. If everything comes on, the problem is the t-stat. If not, you'll have to check the transformer in the indoor section.I tried to turn on my heater recently and haven't had any luck. Tried switching on the AC instead to test it out and nothing either. Had the gas company come out to check pilot light and that was wasn't the issue. The gas company tested my thermostat and suggested I try replacing that. Well I did that and still no luck. I have a thermostat upstairs and one downstairs. So now new thermostats.
I have a Lennox G27M series furnace. Read somewhere that maybe its the circuit boards that needs to be replaced. Some said to clean the sensors in the furnace. When I unplug the furnace and plug it back in, blower turns on and then stops. Green LED lights blink slowly on the control board.
Any advice would be appreciated.