What to do if the furnace or air conditioner won't turn on?

Check breakers first. Then check to see if there is a disconnect switch on furnace (usually a 2" x 4" silver box attached to furnace) and see if fuse is blown or switch is off. Also if you have had your blower door off there is a safety switch for door. Make sure door is secure.

There are multiple reasons that it might not come on but this can narrow it down a bit