Before running to the phone to call the heater guy to decipher the codes there are some thing that you should check. First and most important make sure that the power is on to the furnace. If there are no lights flashing on the circuit board that is a good indication that you probably have a power problem.
Now that you know that there is power, recycle the power on and off. This will often reset the brain of the furnace and wipe out the problem so that it will try to go through the startup cycle again. If the problem was temporary, like low voltage, or low gas pressure the furnace may light up and be fine.
If the furnace stops during the lighting cycle then you may have other Gas Heater Guys and will need to look into the problem further with the help of a professional. Of course you need to make sure that your flue is not blocked or that the filter in the furnace is clean. These things are common causes of problems and can easily be fixed.
These are the first basic steps to trouble shooting your furnace problem. To check out the problem further you may need some specialized tools to find out where the problem lies. Make sure that you at least try these things before calling a professional to save yourself the expense of a service call that is only a couple of minutes long and makes your feel like you are paying the guy to bush a button and run again.