Starter or battery problem, you'll hear something when you turn the key, even if it's a little relay somewhere.. Check all your fuses, the condition of your battery, and that all your grounds are connected and tight. Also check for terminal corrosion on your battery. My GF's truck had the same problem and it was a cracked battery terminal. It could be any one of a number of little things.

Do your accessories/lights work or not? Is the entire car dead or will it just not start?