If the engine won't turn over at all, then are any of the electrical accessories working? And do you hear any clicks when you turn the ignition key?
If you don't hear anything, and there's no power to anything else, then there's a problem in the circuit. First check your connections at the battery for loose/corroded cables, and all fuses/relays. Then you'll have to use a DVOM and trace both the power and ground circuits leading from the bat to the starter, bat to ground, ignition to starter.
If you do hear a clicking, and there's power to other electronics, then your bat can't hold a charge. You'll have to replace it.
And for future references, whenever you start a car every now and then (while you're storing a car for a length of time), you must let the engine run for at least 15-minutes. This gives the alternator enough time to charge the battery fully.
Monday, February 22nd, 2010 AT 8:27 AM