As long as it stayed dry I wouldn't worry about it. Ammo doesn't like to be heated or cooled rapidly, it's the sudden changes that cause problems. If it was cold for a while and then warmed slowly, you should be fine. Give it a good look over and if there is any question about a round or two, don't shoot it.
If the pain is lacking so is the discipline...
"the only 911 call I need is chambering a round" - Mr. Muller, MO car dealer
ive never had issues with such things. but if ammo is going to be kept in poor climate controlled areas for very extended periods of time (as in years) you may want to invest in some primer and bullet sealer to keep the weather and humidity out of the case.
What you may notice in real cold weather is that your velocity is down. Some powders are worse than other powder. That is because all chemical reactions are suposed to slow down by 1/2 with every decrease of 10 degrees celsus. I have never noticed any decrease in velocity in the field, but I am sure I could show it to you on the cronograph. Where people get into trouble is when they work up a max. load in 70 degree temps. and then try to shoot that ammo in 90 degree temps. Presures can get very dangerious. That is the reason I always try to work up max. loads in the hottest part of the summer.
I have had ammo I know has been at below freezing but because I strive to keep as much of my ammo in old ammo cans that are in good condition with good seals Iv'e never had any problem. After many years of storage the old ammo has never let me down. I try to load the cans when the humidity is low and temps around 65 to 75 degrees, if it were really humid I would probably ad dehumidifier packets to help absorb excess dampness. I also store primers and gunpowder in separate ammo cans. These ammo cans are stored in the coolest, driest area I can find. I have powder that's over 30 years old that smells and looks like I just bought it, no acidic smell or fine rust colored powder which are indicators of it decaying. Haven't had a primer fail to fire either but I have had a brand new Remington 30-06 round that did not fire.