What causes my vehicle not to start?

I have a 2001 escape, that will intermittenly start. Seems the fuel pump will engage sometimes. The fuel pump should make a noise when you turn the key, but just before the motor turns over. In my case when I turn the key if I hear the fuel pump the car will start. If I don't hear the fuel pump, it will not start. Do you hear the fuel pump priming every time you turn the key?