Have you tried clearing the CMOS? How did it happen? Have you changed some settings in BIOS?
I cleared the cmos multiple times as well as removing the CMOS battery multiple times. It happened when I was changing some settings in the BIOS, and I changed my ram. It was saying that I was running my ram at 1866 instead of 1600 and I wanted it to be 1600. So I changed it back to 1600 and after rebooting that's when it happened.
edit: i've checked every memory slot with each memory stick and still no avail. Plus I've even tried booting without the memory in their slots (which probably wouldn't help, but it doesn't hurt trying?)