I had my central AC on for most of the week due to the warm weather. It cooled down a bit, so I turned the heat on my thermostat this morning. The furnace kicks on, but then kicks off after about 30 seconds. I hear a click and then nothing. The blower is not turning on. It did that a few times so far this morning. The pilot is on. What's wrong? It was working fine before I turned on the AC earlier in the week. Help!