Yesterday, I came back home after 2 days of absence. I tried to start my pc. As soon as I plugged the pc in, I have noticed that it has automatically started to run. Unfortunatelly, it has only started. It stucked just after that. I couldn't turn it off anyhow, I have tried pushing reset, power, or holding power for 3 seconds, but the pc seemed not to respond. The only solution to turn it off, was to unplug it. Since then, every time I plug it on, it happens again. Fans are working correctly, as well as the power LED. The HDD LED, lits for a split of a second at the very beginning, but then it does not. Although I can hear at the beginning the sound of my DVD-ROM, the open button does not work as well. I have tried plugging off my HDD and DVD-ROM, as well as network card, USB ports and the front panel. The result was still the same, what means that the buttons are fine, and the problem is either in my motherboard or power supply. But then I checked the power supply to other pc, and it worked properly. I have also tried resetting CMOS and swaping RAMs. Still no result. I have Gigabyte GA-K8NF-9, and my CPU is Athlon 3000+, and my power supply is 400W.
In the last few weeks, after coming home from a weekend(and my pc was unplugged for about 2 days), I had difficulties in starting my PC. After plugging it in and pushing the power button, it seemed to start for a split of a second, but then it turned off. Just the power LED has lit for a moment. Solutions for this was a quickly pushing of the power button. It has been happening ONLY if the PC was unplugged for a longer.
A short on then off cycle when first plugged in is normal. Not stating after the power button is pressed is not. First it is not the best practice to unplug or remove power from the PC when not in use. Try removing the CMOS battery and replacing it with a known good new one. When unplugged, the CMOS relies on the battery to stay powered. It is intended for back up not primary power. Your practice of unplugging may have cause the battery to go bad.
I have tried 2 other different batteries, and checked the one from the broken pc in other one. Even with those batteries from the other pcs it doesn't work. The battery suspected to be wrong, does not cause any other pc to crash. I have checked the DCV of all 3 batteries, and the battery from the not-working pc has about 3.1V, one of the batteries I have tried has 0.37V, and the other one has 2.95V. None of them cause the pc to start working. I was considering whether it is possible, that by the time the pc was unplugged, the battery had loaded out, and it caused the CMOS to apply some default settings(this could explain the fact that the pc turns on just after plugging it in, as there should be such an option in CMOS), and those default settings, collide somehow with the hardware or whatever, and the pc doesn't start.