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Take the board out of the case, place it on a piece of cardboard with just the processor/heatsink, 1 stick of memory, and the GPU installed and hooked up to a monitor. No drives, nothing else. Hook up the PSU, and try to boot by shorting the power switch header pins with a small screwdriver. You also need the speaker attached so we can hear any post error beeps. Be sure to reset the BIOS with the jumper too.