Are you handy??? You could try blowing it out with some high pressure air, but likely won't work. Is there a number plate of some sort to indicate what the furnace brand and/or type is. I'm guessing a BRC10...
Does the pilot stay lit? When you turn the temp knob, can you hear the gas valve make a click or ping noise (once when turning it to high, then again when returning it to low setting)? When you turn the other knob from pilot to 'run' (or whatever it says) does it try to start at all??? If it...
Don't forget the city water inlet! Once you have flushed antifreeze through the entire system, shut the pump off, open the hot and cold kitchen taps and gently press on the little button in the city water inlet. (You may have to remove a screen to access the little button.) The city water inlet...