Don't have heat?

Before calling, be sure to check the following:
Is the thermostat turned up and/or switched to heat?
Do the batteries in the thermostat need to be changed?
Is the switch or breaker turned on?
Is your gas turned on, propane or oil tanks have fuel?
Are your vents clear of snow & ice?
Are your filters clean or do they need to be changed?
Are the doors installed on the unit?
Are there lights blinking on the furnace with an error code message?