I have a Lennox RTU, model number GCS16-036-90-2P, it has replaced the control unit (Johnson Controls) couple of years ago, and replaced the blower last year, but again, this year, it didn't light up.
After turn it on, adjust the temperature, I can hear the clicking, then the blower start to run, I didn't see the furnace light up.
I guess it may be the problem from either the ignitor or the ignitor control, if you have any experience of how to identify the problem, would you please share it here.
As biggles stated make sure you have gas. Sounds like the spark ignition is working so you're dealing with dirty burners and orifices,ignition module not powering gas valve or a faulty gas valve. Checking to see if there's 24v at the gas valve or not will confirm or eliminate the module as the issue.