My programmable tstat battery died last night and I replaced it this morning but can't get the heat on my furnace to work. It sounds like it is trying to come on and when I switch to fan only it blows - just not any heat. I've tried fippling the circuit breaker. I took the cover off and flipped on the only on off switch in the furnace I saw but nothing. It has an electric ignition on gas furnace - Goodman GMT power vented multi position gas furnace is the name on the manual.

Did you install the battery correctly?
Is the battery you used for replacement new?
It might not be the thermostat but the furnace just happened to fail?
Did you knock and wires loose when replacing the battery?
Check the peep site or the circuit board and see how many flashs or blinks it is showing for trouble. Then look on the back side of the blower for the answer to the number of blinks which will send you in the general direction of your problem.

Yes batteries correctly installed and new. I don't think any wires were knocked loose.
I looked through the peep and to me it looks like the light is flashing steady but nothing in my manual for single flashing light code.
The furnace did cycle on three times and then locked out before the flashing started. Sounded like both the blower was on just no ignition. Not sure if that means no gas is being released or the ignitor isn't working. How can I tell?

Could be a coincidence,but some tstats have a reset button accessed with a paper clip.

You should get:
Inducer (purge)
Delay
Ignitor on
Pilot
Ignitor off
Main burner
Delay
Blower

Ignitor could be spark or glow.

Reset button was good idea actually but no fix. It has got to be my ignitor. I just hate to call someone because sounds like some switch was flipped in the furnace when the thermostat went dead overnight and I feel like if I could just find the switch but the manual with my heater has zero diagrams so I have no idea what anything inside the furnace door is except for the few obvious things.