Did I just break my computer?

My guess is that something shifted while moving the box. The cold would not inhibit the machine from booting unless it was quite a bit colder, at least in my experience.
My computer workshop is located in the garage, non heated and separated from the house. I work in there year round (In Colorado) and have never encountered a machine that wouldn't boot due to cold. It is about 10 outside now and I have a machine in there that will boot right now. So forget about the cold for now.
You say you "Reassembled" but it sounds like you mean you hooked all your cables and peripherals up. I would recommend you check the seating on your memory, and other components first. I think something prolly came loose and you should be back up and running in no time.
My guess, you need to reseat the ram.
gl

It all sounds good except for the ram part....i think its one of the least possible unseatings.
And like I said while the cold itself wont hurt it, the going from cold to warm can and will cause condensation. which COULD(not will) cause problems possibly very bad ones.

perhaps if someone was dicking around in there and didn't seat the ram properly in the first place...I've seen it happen plenty of times. And the boot sequence stopping each time, sounds like a ram check failure to me. Or cpu. or another dislodged component (sata cable, etc)

In all the times I've moved pc's from the warm house to the cold garage and vice-versa I've never had a condensation problem.

I suppose he could wait until the morning and try to reboot then. Stranger things have happened. But when moving a pc it seems more likely something got bumped loose rather than condensation, in my opinion.

Well, I think the locking mechanism on cpus keeps them in place pretty well but its possible. I had a cpu pull out once when I was trying to remove the heatsink, freaked me out but it didn't seem to be damaged. That was some sticky-ass thermal paste

Did you do anything before moving the pc? Update drivers or change hardware? Are you getting any error messages or beeps?

Starting to look like condensation is not such a long shot after all. Do you live in a humid region? If that's really it I am going to feel quite humbled..

I would eliminate all things mechanical first though. It sounds like the power supply is working. Any way to get into the bios?

I didn't do anything before moving. I live in washington state and I brought the computer outside where there was 1ft of snow on the ground. If condensation did mess something up how would I know what? I can't get into the bios.

I put the computer in the car immediately after bringing it outside and left it there for 3 hours.

Know what you mean, cliffro. It's real cold where I'm at right now and my computer room is great for OC too 'cause the house heater can barely keep up...

Back to the thread, and I've got a theory. Noticing the current weather in Washington State, I would think that if you left that computer out in your car for 3 hours, it would be the same temperature as the ambient--way below freezing. So if you brought it back into the house and tried to power it on right away, then my guess would be that some delicate circuit somewhere just snapped from the sudden expansion / contraction--where that would be is anyone's guess.

One of the likeliest culprits could also be somewhere around the CPU. I noticed that you've got a tower style heatsink, and in bringing it back to the house, that big weight could have cracked something on the mobo or maybe even damaged the CPU itself (as a result of the extreme cold making all the parts more brittle).

Unfortunately, me thinks you're toast.

OK, I take back that part about you being toast...

That was just a theory that came to mind when I read this thread. The others are probably more correct than me, but look at it this way too. My CPU idles in the low 70s and other key parts (mobo, GPU and HDD) idle at higher temps. Powering on jolts the circuitry from 33F to 70 minimum, which is about a 40 degree difference--could be enough to snap something delicate.

Just a thought...