Eight things you can do on your own to save yourself a service call:
Is the power and/or gas on?
First make sure there are power and gas to your home. Power outages occur, gas services can become disrupted. You don't want to pay a contractor to come out and tell you there's nothing he can do until utilities have been restored.
Is the circuit breaker on?
Next, check to make sure that your circuit breakers are in the on position. If the breaker has tripped, reset it by pushing it all the way off first until you hear an audible 'click', then all the way to the on position. A note of warning: circuit breakers are safety devices and they trip for a reason. If it trips again after you reset it. Have it looked at. The underlying cause needs to be addressed.
Is the thermostat set properly?
Check your thermostat. Try replacing the batteries if applicable. Make sure that it is set to cool if you want AC, or heat if well you know...Also, very important: unless you want the fan to blow continuously, always keep it set to auto. Just a note of caution to DIYer's: we are all for you saving a buck or two if you can handle the job yourself (safely). However, we have changed a lot of expensive control boards after they became fried because someone thought they could change their own thermostat. They didn't disable the power first, touched the line and neutral thermostat wires together and well you know the rest.
Is the filter clean?
You can save yourself money by changing your filter frequently! We cannot stress that enough. If your filter is blocked, your system has to work very hard to draw air into the return and out through the supply registers. It DOES cause premature mechanical failure, very high electric bills and unnecessary visits from your favorite HVAC contractor.
Is the condensate line clogged?
Is your condensate line blocked? If your system was installed with a safety float switch, your system will shut down if your condensate line is blocked and your float switch is functioning properly. Typically you will find a 3/4" pvc condensate drain near your condenser (outdoor unit).
If it's not outside with the condenser, it will typically terminate inside the bathroom sink cabinet closest to your air-handler/ furnace. See if you can clear it out with a wet/dry shop vac. If so, your float switch should reset itself and your back in business. Sometimes the clog is really bad and we may need to come to the rescue with a high pressure blast of nitrogen to clear the clog.
Is the furnace power switch on?
Near the furnace, you should find a switch. It looks like a light switch. Make sure it wasn't accidentally turned off. Sometimes there won't be a switch, but a 'pigtail' (power cord) plugged into an outlet. Make sure that it's plugged in.
Is the condenser disconnect in the on position?
Near your outdoor unit, there should be a metal or plastic electrical disconnect. We don't recommend opening the disconnect unless you are confident working with high voltage (220 volts or more). If there is an on off selector switch that you can visibly see on the side of the box, make sure that it's in the on position.