Reparing a furnace?

Your gas valve is bad or there is a loose connection preventing voltage from reaching the gas valve solenoid; or a safety switch is preventing voltage from reaching the gas valve.
The problem could be a faulty circuit board/ignition module(a little gray or blue box that all the wires go into)
You need to be more specific with your question as this is all the info I have time to give you without writing a users manual with the trouble shooting guide for you.
If you are not experienced with troubleshooting a furnace you need to call a reputable, licensed and insured service company. If a part needs to be replaced plan on spending $150-400 depending on the part and how much the company is willing to take your pocket book for a ride.
If your furnace is less than a year old then it is still under warranty for parts; plan on paying for the labor.
If you have your owners manual there is usually a trouble shooting guide in it and you might be able to find out the problem yourself.
Good luck.

Duhanlorian
June 2008