2. Is the gas meter turned on with your local utility provider?
3. Have you made sure the electrical power to the house is on?
4. Are there batteries in the thermostat? – If the thermostat display doesn’t light up, change the batteries.
5. Did the circuit breaker get tripped? – Reset the breaker, if the breaker to your HVAC unit has been tripped. Keep in mind that circuit breakers trip for safety reasons and if the breaker trips again, a certified electrician should look into the problem. Many times, the breaker isn’t the problem, it’s something in the wiring pulling more electricity than it should.
6. Check the condensation pump and/or A/C drain line? – If it is full or plugged empty the pan and flush the drain line. An HVAC technician IS recommended for this.
7. Have you made sure the thermostat is set to cool or heat?
8. Is the outdoor unit dirty and/or filled with cotton or debris?
Please note that articles from other places may mention more or different things to check, however, they most likely are not safe to check yourself. I just read an article where an HVAC technician died by touching the filter cabinet. They died just by trying to change the filter. Change a filter is normally a safe thing to do. If you ever question your safety when checking your HVAC equipment, call an certified and licensed HVAC company.