This happend to my old mobo my BIOS settings where set wrong so i changed them back and that did fix it but then it keept happening as the BIOS keept changing back have you tryed to keep reseting the computer untill it loads up? if not
then try the power button on ur case if your computer if turning on/off check the case button it might be thst turning your computer on/off as it has happend to me
in the end i just bought a new computer