Its about 15 degrees here and my car will not start. i have 2004 mercury mountaineer. It started last night when it was only 9 degrees. What could be wrong? Antifreeze maybe? and if so where do i put it in?

Not starting can be alot of things inculding frozen fuel lines or a fuel pump that so cold it won't turn. If it's cranking just fine you might try knocking on the fuel tank to shake the pump. with someone trying to start the car knock on the fuel tank and listen fort the pump. If the battery is too low the pump might not start but the engine may still crank. It might start if it warms up outside and you'll never know what the rerason was. Keeping your fuel tank above half a tank is a good idea too. Try to connect and jumper car to yours and let it charge the battery for awhile then try to start it as the extra voltage might be enough to start a cold pump.