In reply to RockAngel: When the lights come on, are they full brightness, do the headlights work fully? If they don't, then it's likely to be a battery issue. If the car seems to have full battery and all lights work fully, it could be an immobiliser issue(if the car has one).
If everythings looking good, no immobiliser fitted, won't turn over and NO noise when you turn the key(no click from under the bonnet), the next place I'd look is the solenoid, as they stick. WHat the solenoid does..... when you turn the key the solenoid is energised by the battery, it moves to engage the starter motor onto the flywheel(engine), as it moves it also works as a switch to put electricity to the starter motor. When you release the key after the engine starts the solenoid(it's an electro magnet), moves back to disengage the starter motor from the engine and at the same time cuts off the electricity to the starter motor.
If you try to start a car with a flat battery, the solenoid can move and then get stuck part way as it doesn't have the energy to fully fully enagage. This is when moving the car a few feet, in gear can release it. I've had to hit the solenoid assembly with a hammer or wheel brace in the past to clear it. Then, you need a fully charged battery, or partially charged + jump leads and the car should start.
It could be a lead has corroded or chaffed / broken. You need more than an internet forum to help with that though. Mechanically minded friend is best bet.
PS. If you've run the battery fully flat in the past, and you've done it again recently, it could be the battery is totally nackered. They really don't like it.