Why won't it turn over atleast?

My bronco will not turn over or run. I have tried replacing the cylinois, starter, battery, ignition switch, and also found out I didnt have a nuetral safety switch either. When you turn the key over you hear the fuel pump strat up and all of the lights come one, but nothing. It won't even turn over. I have called numerous people, and gotten different advice, but nothing seems to be working. What could this be from?