Ruud Silhouette furnace will not ignite. Blower continues to blow cold air, no heat.

Tried adjusting thermostat temperature to lowest setting and turning off heat control.
Flipped the circuit breaker on/off
Turned off the gas ignition switch and back on
Changed the air filter

How do I Install a Ruud Furnace Ignitor? | eHow

If the ignitor fails to come on you could have a bad thermostat, need batteries in it, have a lockout due to a safety etc. What code does it show on the control panel-count the blinks and compare to the troubleshooting chart on the inside of the air handler door.