Do You Know Why The Blower Is Not Starting On My Furnace?

I have a repairman coming over this morning to have a look. Last nite I could hear my furnace running, but the fan did not come on. Since I have replaced the surface ignitor about 5 times since I moved here, I figured it was acting up, so I called my handy dandy brother who came over and told me thats not the problem. While he was looking at the furnace, the fan motor kicked on and I assumed it was my imagination until I woke up at 3 AM to a 45 degree house. I could hear the furnace running down stairs, and the blower didnt kick on. I jumped up and shut it off. What do you think is wrong?