Might need to clear CMOS if you changed something in the BIOS (switch power on the PSU off or unplug it, push the power button a couple times to clear all energy from the system, and switch the CMOS pins from 1-2 to 2-3 on your motherboard for 5 seconds. Switch the pins back to 1-2 and try to boot then. The numbers on the pins may vary from motherboard/manufacturer)