Because you overclocked it. turn the power off on the system.
You will need to re set the bios of the board to its factory settings.
Open up the tower case locate the battery and near it will be three pins with a jumper on two, move it over so the other two are bridged by the jumper, then put the jumper back where it was, all done !. Should boot fine.
But then you will have to go into the bios and change some settings Ie: HD interface Sata mode, or Ahci mode. Keys to enter the bios can be F1,F2 or the Del key of the keyboard just after the bios splash screen.
Your Bios reset pins are located just next to the main Led,and switch connector header block of the board, the little black jumper top right above the white block.