Since what everyone else has suggested hasn’t worked, there’s 3 more things to test:
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Update your BIOS. Depending on the age difference between your CPU and your MOBO, the BIOS might not be configured correctly for your CPU and thus half your RAM is unable to be addressed.
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Test your RAM sticks and memory slots individually. Put your sticks in the primary channel (per your MOBO specs) one at a time and reboot to see if they’re actually working. Then try moving them around to see if the issue is a bad slot, rather than a bad stick.
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Are you sure you’re not running a 32-bit OS? You’d be capped at 4GB system memory on 32-bit Windows, for example, no matter how much physical RAM you have.
I’m inclined to agree that the CPU might have been set incorrectly / have a bad pin, especially after the black screen and BIOS errors after updating.
Unfortunate, but it happens to the best of us.