Many reasons out there:
*)I can tell you that if it's a new computer, the problem can be software related/viruses etc.
*)CPU operates at a high temperature mode than allowed so the BIOS will automatically shutdown it. But it depends on the type of motherboard and processor.
*)Any recently installed hardware like RAM's and add-on cards that are not 100% compatible. You've to uninstall them for debugging.
*)In case of old motherboard, it happens because of power supply issues. First you need to check the SMPS, then supply regulators on the motherboard probably FET's,diode etc. Those FET's works on a high frequency mode, so they can be bit costly and need to replace exactly thesame one.