Why doesn't my engine make any noise or start?

Yesterday I drove around for about an hour then came back home. Everything seemed
to be fine as I was driving it until I tried to park. I shifted my car to Park but as soon as
I took my foot off the brakes the car rolled forward. So I put my foot back on the brake
and it stop rolling, I lifted the emergency brake and took my foot off again and the car
still rolled forward. I was in the car with a friend who told me to switch places with them
so they can see what's going on but since I didn't want the car to roll away I turned it
off, which was a bad idea on my part. Now the car won't turn back on. When I put the
key into the ignition and turn it the air, radio and lights turn on but there's no noise from
the engine at all. The lights that come in on the dashboard are the Air Bag light and the
Check Engine light. Any ideas to what's going on and how much it would possibly cost
to fix it? I own a 2002 Chevy Cavalier.