My furnace won't start up? ...
Answers: you didn't include many details. is it gas or electric?
If gas do you enjoy a pilot light? if follow the directions on the service panel and try to re-light it
Sometimes a heavy twirl storm can blow them out
If it's electric, make sure the power is on and you don't enjoy a blown fuse, or tripped breaker.
Mine wouldn't start a few years ago ... a child had plugged the pipe that runs outside next to a golf ball. The serviceman said it happen all the time, but it's usually bird eggs.