Central Air and Furnace not working?

Check the circuit breaker to the HVAC. The transformer may be bad. It is the component that reduces the voltage to dc that goes to the thermostat. Usually located in the attic. Perhaps, since your husband was able to install the new t-stat, he could check the voltage at the wires going into it ? No voltage with the breaker on would point to a broken wire or bad transformer.

Calling a service tech is expensive and may not solve the problem. Techs do not benefit when things work, only if they break. I had to get 2 different companies to fix my AC this year because the first guy was an idiot, and I was afraid of damaging the compressor motor.

If you have an electrical tester check the thermostat. Keep the stat in heat and see if you have 24 volts from R to Y. If you do you have control voltage, now check from R to W if you read 0 volts then the t-stat is closed and you should have heat. If nothing works I think you have a control voltage problem. R power Y cooling G fan W heat on tstat. If t-stat has voltage and R and W is closed you have may have another electrical problem or the unit has tripped out on a safety, then you will have to call for service

Go back to the circuit breaker box, find the breaker you need, push it towards on, then flip it off (make sure it clicks) then turn it back on...

If the fan is not working in air mode or fan, it is an electrical problem

if the fan works on air or fan, but not heat it is something else either the pilot or thermocouple.....

check the fuse on the unit and see if its blown