When I was using the computer, suddenly heard the sound like turn off, then the monitor display no signal. I cannot do anything except power off the PC by push the power button. When I reboot the system, everything is working fine. However next day I got the same problem again. It causing me lose not saving data. When I reboot the system, everything still working fine.

1. I am sure not monitor problem, not video card problem, not os power management problem.

2.Is it motherboard or power supply problem?
When I heard the sound like turn off, then monitor display no signal, but I can still see the PC green light on, is that mean the ps is good, or a bad ps still supply a little power.

Is this a recent problem? Have you had this problem before, even just once in the past?

Check all cable connections and the condition of the wires also. Make sure none are being pinched by the enclosure, there are no cuts in the insulation, etc.

Are you on a power bar? Check if the power bar is faulty, by plugging the PC directly into the wall outlet.

To determine if it's the power supply, you'll need to find a replacement, borrow from a friend? grab one from work? a second computer?