The heat or air will not come on

1st check breakers or fuses to the furnace
2nd check wires both tstat and high voltage wires to make sure connections are good and no critters have chewed tstat wires into
3rd. Not knowing what kind of furnace you have , if it has a circuit board/control board check if it has a fuse in it (automotive type) that could be fried
4th. With a multi meter check the transformer for power going in and power coming out.

Let us know what you find