I put together my 3rd computer successfully, installed all the drivers, and noticed I must have put the Hard drive LED plugs (from the case) in the wrong way, as the light was very dull. While the computer was on I unplugged the HD LED cable (stupid I know), realised what I did, and shut off the entire computer. When I booted it back up, the entire computer zapped, the lights turned on for a split second, then completely died. I thought for sure the MB was fried. Wouldnt turn on even after countless tries. I unplugged EVERYTHING, replugged, and tried again.. still nothing.
I took out the MB, disconnected everything but the RAM, CPU +heatsink, and graphics card... plugged in MB power, CPU power, and PCI-E power, and tried to boot up using the same PSU (OCZ 600w modular). Nothing...
Thought it may be my PSU, so I grabbed my corsair 750, and tried the same thing. The MB BOOTED!!!, I thought, well, atleast its not the MB and all components attached. I hook up the OCZ 600w back up to the MB, CPU, and GPU (outside of the case on cardboard) just out of curiousity and the damn thing booted.
My question is, how on earth can that happen? It doesnt make sense to me. I tried several outlets, so the power coming into the PSU wasnt the issue. Is there some kind of safety on the MB or in the PSU that cuts all power for x amount of time? Did my unplug of the HD LED and replug cause this? Maybe a faulty PSU that will likely fail soon? Thanks for all the help.
Unplugging the HDD LED cable can not cause this problem. I am intrigued, though that you say that LED was "very dull". In my experience, it either lights and flickers at a "normal" brightness as the HDD is used, OR it does not light at all if it is plugged in backwards. Was it plugged into the wrong pins on the header, perchance?
I had a slightly similar experience about a year ago. Ultimately the problem became clear - a failing PSU - but that's not my point. My point is symptoms. My machine would suddenly quit. When I pushed the front panel button it would do nothing - no response. BUT if I unplugged the power cable from the back and left it disconnected for 20 to 30 seconds - maybe a minute - then reconnected, it would start up and run for a while. I found that, rather than disconnecting the cable, I could get the same result if I merely turned off the rear PSU switch for a minute, then back on. I theorized that somehow the small part of the BIOS that is still running when "off" was caught in a loop. Removing all power for long enough allowed the residual charge in the PSU's output capacitors to be discharged so the BIOS lost all external power, and reset itself to the "cold boot" state so it worked until the next PSU malfunction. MAYBE that is what happened to you. The first removal of power to test the machine was brief enough that it did not fix itself. But a later longer disconnection DID allow the cold boot clean state to be set, and it worked thereafter.
So, what is the real cause? Could be bad PSU or bad mobo. You have it reassembled with the original components now, so see how long it works. If it fails again, try substituting a different PSU and see if that solves the problem or not.
So regardless, something is wrong. I dont think flukes like this even exist with computers, so I think youre right when you say the PSU or mobo is failing. The HD LED plugs may have been plugged into the wrong LED or wrong little prongs that are cluttered together. I dont remember.