Did the 2 missing cores start appearing before or after you decided to overclock? If they disappeared after the OC it sounds like 2 of the cores may be dead! Congratulations, however, as your CPU is somehow still standing. If you already tried to reseat and repaste the processor and its still showing two cores, you could try flashing the CMOS or updating the BIOS, but it sounds like components on the processor itself could be french toast.