Lost of power can be a problematic one to fix. While you think you have a good PSU, it may not be. Further, the mtrbd. does have a logic power trip as in its always active once power is present no way for user to interact with it. The best example of this in action is when you do a system shutdown, that's really a logic action, not a real "turn-OFF" the switch sorta thing. Its work in this manner during start-up.
I suggest, return AC plug and wait a few minutes, then power-ON. If the system refuses to boot, re-try the shortening power pins just for amoment, it acts as if the frt. panel button was depressed. Don't keep the short for long, it is a momentary action. Of course I exclude any other possible issue here as you may have another area at fault. But, once the system starts to reboot, don't unplug it again, it should behave now. IMHO, I don't trust that PSU, its FSP(fail safe protection) could be faltering and some cap or such is retaining enough juice or not and it thinks power is being applied too quickly as in outside AC problems. Also, the bios could be set to not turn-ON(reboot) after power fail, it has to wait and only turns-ON via the user hitting the power button. A quick power-ON by the user maybe interpreted as a power problem.
I've experienced this problem before. Sometimes after powering ON or NOT!, I use the small sw. found on some PSUs and turn-OFF then. It usually works after I return power-ON again after a 3-5 min. wait. Understand also, some cpus and overall demanding power-ON demands can exceed a weak PSU, though repeated OFF/ON seems to clear it. The power demand is too much for some power leg of the PSU and it doesn't deliver. Replace with a better wattage PSU or name brand one(quality).
This is a tough one. After moving my computer to a different room it would not turn on. Press the power button and nothing happens. I should say now that i build computers and pretty much know what i'm doing. So i assume that somehow my power supply or mobo has gone bad (after checking everything is still plugged in properly). I pull the power supply and short the PS_ON to ground and check the rest of the plugs and find they all are putting out the proper voltages. So it must be the mobo. So for kicks i unplug all the peripherals from the mobo and plug the power supply back in. Then use a little piece of wire to short ps_on to ground and the thing comes to life but as soon as i take away the short it dies again. BUT i notice now that the ethernet jack has it's lights flashing (they weren't before indicating there was no power to anything). So i put everything back together (WITHOUT unplugging, i know i know but it's neccessary) and press the power button. It boots up just like nothing ever happened. I turn it off and back on and everything is good. Then i unplug from the wall and plug back in, press the power... and it's dead again(ethernet lights aren't on either). So i short the ps_on to ground manually again, everything comes to life but dies when i remove the short. But now it turns on when i hit the power button. So what is going on here? The motherboard seems to do some kind of reset when it loses power and it no longer recognizes the power button (i did try manually shorting the mobo pinout for the powerbutton, it didn't work either)? So it's a fairly new Gigabyte mobo very standard specs. Intel Dual Core. No onboard graphics. Nice Liberty power supply.
Any thoughts would be appreciated.
Please be constructive.