Just went through this..
Where do you live? I travel a lot for work and regularly leave the car sitting M-F. Two weeks ago it was below 10° all week and when i got back the car wouldn't start but had power to run locks, lights, etc.
Can you drive around for 45+ minutes without problems? If so, google how to use the dash test on your alternator. The battery should be 12v+ with the car off, 13.7+ with it on. If both the above are true, probably not the alternator..
Next test the battery. Either take it to Autozone and they'll test for free, or if the battery isn't fully charged they'll want to charge it first before testing. Either give it to them for an hour or take it out and throw a trickle charger on the posts over night.
Let the battery sit (out of the car) for a day or two once charged and then drop it in and test it. If it works, you're good and it was just cold weather. If it doesn't, your battery isn't holding a charge and is cooked.
Last scenario is bad. If you charge it and let it sit and it goes in and works, the battery is fine. If it dies again after a day or two, you are leaking voltage and have a real problem.
Best of luck!
Sent from my DROID RAZR using Bimmer App
///M3 VERT, COLD AIR INTAKE, CF FRONT SPLITTERS, HALO PROJECTORS WITH CLEAR CORNERS, 15MMFx12MMR SPACERS, FRONT STRUT BRACE, ROGUE ENGINEERING POWER PULLEY KIT, BLACKED OUT KIDNEY / SIDE GRILLS, SILICONE INTAKE BOOT, ///M-STITCH SHIFT BOOT / HANDBRAKE BOOT, MEGAN RACE EXHAUST, ZHP SHIFT KNOB
Last edited by Bmill160; 02-14-2013 at 01:09 AM.