Take it down to a minimal configuration, i.e. 1 CPU and 1 bank of memory.

Swap different components in until you get a minimal working system.

Then add components back until it stops working to isolate what may be bad.

I know it's not what you want to hear but it sounds as if your CPU isn't available to the boot process.

I'd guess it's either died or isn't seated properly on the main board.