Before placing a Service Call, try this checklist:
Make sure there is a call from the thermostat and set properly.
Make sure no emergency switches are off (including the furnace switch). Furnaces in basements may also have a switch at top of steps by the basement door that gets mistaken as a light switch!
Make sure the breakers for the indoor and outdoor equipment are in the "on" position - reset if necessary.
Make sure the outdoor disconnect is on. Some have a pull-out plug that can be put in upside down preventing the unit from starting. Some have internal fuses or circuit breakers. If you know how to check fuses you may do so.
If you have a condensate pump with a safety switch, check to see if the pump is completely full of water. If so, make sure the pump is plugged-in and the vinyl tubing isn't clogged, (it could be a bad pump). Sometimes the end of the tubing runs outside and gets stuck in a mulch bed or plugged with dirt. Condensate pumps can usually be found just above the floor, against the indoor unit.
If your outdoor unit has a "Reset Button" press it. If that was the problem and you have to press it a second time, then there is a problem and a service call will be needed.
Your unit could have a safety device locking it out. Reset it by turning the system off at the thermostat or breaker, wait 1 minute then turn it back on. Wait up to 10 minutes to see if the outdoor unit starts.
Make sure there are no leaves, snow, etc., blocking the outdoor unit or intake and exhaust vents.
Make sure you have sufficient fuel supply (for oil and lp gas-fired equipment).
These are the most common problems a homeowner can check.