I had a similiar problem a year or two ago and it turned out to be the fan control sensor, or something along those lines. I am not sure exactly how I diagnosed the problem, but I am sure it was from searching the net on some common problems and troubleshooting techniques... The part was cheap and easy to replace.
I vaguely remember just bypassing the sensor to see if that would get the igniter to kick on.
I believe the way most gas heaters work is that they will turn the blower motor on first for a few seconds to make sure any (or as much as possible) gas in the unit is cleared out before it turns on. Dont want the thing blowing up afterall, right? After a few seconds you should then hear the igniter clicking, etc...
You should be able to run the unit with the access panels off just to see if you are getting a flame or not. Of course dont do this if you smell gas, dont do it unattended, and use common sense. If you do hear the igniter but see no flame, that might be your issue.
Also, any chance you turned off the gas supply to the unit after last winter?
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