Computer won't turn on

a bit of technical knowledge for you to maybe understand why there's a light on your motherboard, (so therefore power flowing through the power supply), but it won't switch on.

for many years when pc's were first invented and mass produced, the power supply was a bog standard tranformer/power brick, but with a handful more wires coming from it. the switch on the front was put there for ease so that you didn't have to reach around the back of the pc to switch it on. to put that button there though, you had to run a mains voltage cable through the case to the front. which was obviously somewhat lethal to high-precision electronics. it was the only way though.
then came along ATX. this put a different light on how power supplies worked. they were more intelligent, and had more control. the motherboard also had control over the power supply, so it could switch itself off after windows/linux/<your desired OS> shutdown properly. this was called ACPI. not sure what it means, but it's basically CPU-controller power, motherboard wide.
in order for ACPI to work, the power supply has to remain switched on. the motherboard is constantly receiving power from the supply, even though the pc is switched off. the power switch on the front of your pc now only has 2 thin wires, and they go to the motherboard now, not the power supply itself. the motherboard became the "middle man" to switching the pc on and off. pressing this button completed a link in the ACPI controller, and this then "shorted" 2 of the pins on the power supply together. keeping these pins connected together told the power supply to start providing electricity to the other major power lines. the 12v, 5v, and 3.3v (CPU).

for this mechanism to stop working, one of a few things could have happened. most are usually down to storm damage or power surges knocking out either the intelligence on the power supply (therefore providing power, but it's not clever enough to know what the "switch on" signal from the motherboard means), or the ACPI controller on the motherboard. if this happens, when you press the button, you're telling the controller to switch on, but it's not telling the power supply, hence nothing happens.
it's *usually* the power supply. but don't quote me on that, because i have had to replace motherboards because of this in the past.

first thing to try. *completely* unplug your pc. and leave it for half an hour. this will give the power supply a chance to "reset" (yes, as daft as it sounds). this worked before in about 60% of the cases i've dealt with. plug it all back in and try it as normal.
if that doesn't work, then you need to test the power supply. this is quite easy. i will tell you how. unplug all the power cables from everything inside except a cd drive (you'll see why later on). then take the long connector with 20 wires going into it and look at it head on. there should be just *one* green wire. this is the signal wire that the ACPI controller "shorts" to switch the supply on. what you need to do is get a short piece of wire (or a paper clip, or something metal), and put one end into the green wire's connector, and one into a black one (there's a black wire either side of the green one, about 5 or 6 in total). you won't get a shock, it's only pushing about 5 volts down that wire. if the power supply is good, the fan inside will spin up and the cd drive will spin up as it will normally. this will also tell you that the 5v & 12v power rails are good.
*IF* that doesn't work, then your power supply's at fault. go to the nearest computer store and buy a new one. if it did work, and you got lights out of your cd drive, then your motherboard's not working. time to get a new pc.

HTH =)

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