I have a 2001 Chevy Blazer 4.3 4X4. I parked the car for about 30 minutes and then ir would not start. It would not even fire. I tried again and it started. I drove home and it did the same thing. I took the battery and had it checked and they said it was low so I replaced it. When I turn on the key everything works: lights, radio, gauges, etc., but the car will not fire. Any suggestions?