I have a Asus (P5N-D)..Worked fine with 8gb memory then one day would not boot the monitor would not come on.Removed 2 sticks and it will boot just fine...Did those 2 slots go bad..The same memory still works.
troubleshoot each stick individually, put in one stick and boot, run memtest86, if it passes try the next. if all 4 sticks check out, test each slot, usually by inserting them one by one until it does not boot. reply back to let us know how that goes.
odd, have you tried both memory slots individually? usually when a slot goes bad all it does is not pick up the ram on the system. there could be a short somewhere, maybe some dust that's slightly conductive got in there. try some compressed air in there and let me know how that goes.