Furnace Won't Fire - Suggestions?

I have an American Standard 90 furnace. I just moved in a few weeks ago and we fired it up with no problem. About a week ago, it stopped putting out hot air and would only blow cold. I quickly learned that it wasn't firing and thus.....cold air. I pulled the filter and noticed it was completely plugged. I have tried replacing the pressure sensor switch with no luck, tried resetting the main limit switch and the flame rollout switch. I am assuming the furnace overheated, but not sure how to get it to come back to life. BTW, the heat inducer fan powers up until the furnace tries to fire, then it shuts down. I never get a pilot or any flame. Thanks