Computer won't start!

[Asmegin]

So today I built a computer for my sister:

Antec Sonata III
Intel Core2Duo E6750
Gigabyte DS3 (Rev 3.3)
Geforce 8800 320mb

Everything went fine. Tested it for 3-4 hours. Played FEAR a bit. Temperatures were all looking good. I finished for the night and left her to it.

10 minutes later she says she was installing UT2004 and it just shut off. No weird noises or anything...just shut off.

Now it won't start at all. No lights, no PSU/CPU fan etc....

I've checked over all the connections and everything looks good. Did the obvious stuff (different cables, different outlets, checked power jumper etc) but to no avail. Peripherals plugged into the computer via USB DO get power (particularly the wireless keyboard/mouse dock).

Am I missing something simple? I'm leaning towards doing a tear down/rebuild but I don't really want to take to take it that far yet.

 

[Asmegin]

Okay I grounded the green wire of the ATX connector to the black and the PSU started up.

So it's the motherboard?!

 

[azimuth40]

Why pick on the motherboard so quickly. I have seen several bad switches on cases. You bump the case switch shorts and stays shorted, system shuts down and stays shut down. Never do the green and black wire trick with the motherboard plugs connected. If it really is the motherboard as in the CPU supply you could lose your CPU or RAM or both. Pull the switch connector and just short the burg pins with a flat blade screwdriver momentarily. Even safer carry a spare switch with a wire from an old case connected to it, any old switch will do,

 

[cyberjock]

Unfortunately that simple test didn't really tell you much. In fact, I'm not sure if it told you anything useful for you at all. Sure it can come on, but are the voltages correct? Most motherboards have some fail safes on them. They try to save you from doing anything crazy like shorting out the entire board, or a +12v to ground short. They aren't 100% protection, but it is a line of defense. Your PSU has similar features in it. I would try checking out the components that you can test with a voltmeter. Maybe try to take some voltages from the PSU with it unplugged completely, try checking the resistance of your switch, the simple things. Unfortunately if this doesn't work, your last choice is to strip parts out of your computer and try booting it with the minimum to make the computer POST. Although it is possible there's a short to ground somewhere, you could try removing everything from the case and reinstalling it all watching for anything that could possibly be wrong. For instance a motherboard mounting post where there's no hole in the motherboard or a loose screw.

Whatever you do, DON'T try to take a resistance or voltage check across anything that is electrically connected to any of your computer parts. Your PSU is pretty smart about avoiding grounds and stuff, but if you take a resistance across your motherboard, that could be a bad day for you. Digital voltmeters apply a voltage to determine the resistance, and you don't want to find out that voltage burned out a component.

 

[Asmegin]

Well, I've figured it out. A bad stick of OCZ RAM.

I didn't even think it could be the Ram. I've never heard of a bad stick cutting total power to the motherboard....