Half my memory dropped out. What could cause that?

I have a dual socket Intel board. SM X8DTH.

I have six 4GB sticks in the first slot of each channel of each CPU.

That's three sticks for each CPU. NUMA is enabled. They are ECC DDR3 RDIMM

SiSandra shows only banks 1,3,5. 0,2,4 seem to be missing.

Otherwise both CPUs seem to be running.

BIOS only shows 12GB too.

Everything seemed to be working fine until the other day.

What could cause one CPU to lose memory?

Could be a bunch of things, but all motherboard related. Check the socket pins.

Reseated both the CPU and RAM?
What troubleshooting have you tried?

Nothing yet. Just trying to come up with some theories to rank my testing from the easiest to the harder ones.

First and easiest would be to reset my BIOS. Maybe a power surge (we've had some recently) flipped a bit.

Another guy thought maybe the CPU lost it's IMC due to extreme age and previous hard life.

I could move one stick off one CPU over to the other and see what happens.

Every time I run SiSandra's CPU module, my system BSoDs. Every time

I worked all night and need some shut eye for now.

Must have been a flipped bit or I have an intermittent problem.

Shut it down overnight while I was a work to help keep the house cooler during our heatwave and this morning when I booted up all 24GB are back.

Yesterday I rebooted multiple times and got into the BIOS a couple of times, but all I could see was 12GB.

And no, the computer wasn't overheating, just our house and us due to the smokey forest fires surrounding our region.

I'll keep an eye on this to see if it reoccurs. Computers can be weird