There may be a fuse or two on the furnace controller board mounted in the outside access panel. If you hear a clicking sound but the blower doesn't come on, check those fuses. Also check the blower to see if it turns freely. Without the fan running, the sail switch will prevent the furnace from lighting regardless of what you do with the thermostat. If the rig sat for a long time without the furnace running, critters may have built a nest inside the blower housing preventing it from turning. Even if you could get the furnace to light, it wouldn't do any good without the blower running, and you run the risk of overheating or fire if it gets too hot. There is a high temperature limit switch to prevent that from happening, but I wouldn't count on it.