The mystery of my desktop that locks up when it gets too cold
So I definitely appear to have the most ironic of things, a computer that doesn't like it when it's too cold. I don't think it's the CPU fan or the CPU, either, which leaves me strongly suspecting the motherboard. I guess it's time to update to the latest BIOS.
(Sadly, updating to the latest BIOS didn't fix the problem.)
What has been happening sporadically all winter is that when the ambient temperature around my home desktop drops too low, the machine will lock up. When the ambient temperature rises again, the machine boots up again. I have not nailed down exactly what 'too low' is, but based on motherboard sensor readings and an external electronic thermometer, it is around 60 F ambient outside my case and no more than 68 F inside it. Since this is a far lower temperature than I'm comfortable with, it only happens at times when I'm nowhere near the computer. The CPU temperature appears to be irrelevant; I have run CPU soakers that kept the CPU temperature fairly toasty warm and the CPU fan actively working away, and still had the machine lock up.
One time I deliberately created a low temperature situation where I could see the machine in this state after it had locked up, before the temperature rose again and it came back to life. When I observed it in its locked up state, the power lights were on and all of the fans were spinning but power was not being provided to my USB keyboard, and power-cycling the machine didn't bring it back to life (only letting the heat come on did).
(One might wonder why I didn't see this last winter, but as far as I can tell I only assembled and put together this desktop in late March of 2018, which is after the really cold weather that induces such low interior ambient temperatures.)
This is a genuine and somewhat frustrating mystery. I have no idea what the cause is, apart from that it seems most likely to be something related to the motherboard, although I suppose it could also be the power supply sagging the voltage on one or more rails when it gets too cold. One of the possibilities that worries me now is that sufficient cold is making various metal parts shrink and move enough that they're creating some sort of short that shuts things down; this could be a fault in the motherboard, or something about how I put the machine together.
(In theory I could completely disassemble and reassemble the machine, to re-seat and re-do all connectors and so on. In practice I have very little enthusiasm for that, especially taking apart the CPU and the CPU cooler.)