Hello
The problem can be determined by the type of switch off, your motherboard will enter "soft mode" if your PC's CPU reaches a high temperature meaning your heatsink has lost contact with it.
- Soft mode is when the motherboard LED is still lit after switch-off, if your PC switches off again check the LED (green bulb on motherboard), if lit then check your CPU heatsink. It would be a good idea to monitor your CPU temperature in Windows.
If, however, the LED light is OFF it could mean a short-circuit with components, check all cables and make sure there is no copper terminals showing or cut wires, upon a short-curcuit the PSU switches off the power.
NB: I'm not 100% sure if the LED light goes out after a short-circuit, but I know you can't turn the PC on untill you unplug at the mains for about 10 seconds. If you can turn your PC back on without unplugging then it seems like your CPU is overheating
Good luck, let us know how you get on