|Power switches on desktop computers are extremely reliable. |
The only ones I have encountered that actually didn't work were ones that had been physically broken - damage you can see on the exterior of the switch.
Rarely, if someone has removed the case front, e.g. to install or remove an optical or floppy drive, a wire may have been broken off at the switch where that can't easily be seen when the case front has been re-installed.
However, sometimes the case button that presses on the power switch can get stuck inwards such that the power switch is always on. In that case, on a working system, the mboard will usually shut off in about 4 seconds after you press the button (depends on a setting in the bios Setup) , and the computer will not start up again until the case button has become un-stuck and then you press it again. Make sure the case button is not stuck inwards.
If the case button is not sticking inwards, almost always, when your computer does not start up when you press the power button, something else is wrong . A failing or dead power supply is probably the most frequent reason a computer won't boot.
As OtheHill has said, the Power switch for an ATX case / ATX mboard is the momentary contact type - on only when pressed in. If your case has a Reset switch, it is the same type of momentary contact switch on AT and ATX cases - you can connect the Reset switch to the pins on the mboard for the Power Switch, and visa versa, but almost always there's nothing wrong with your Power switch.
Or - you can disconnect the Power switch wiring from the two pins it connects to on the mboard, and do manually what the Power switch does - briefly short the two pins to each other.
Failing power supplies are common and can cause your symptoms.
Check your PS.
They often partially work, fans and hard drives may spin, leds may come on, yet you may get no video and the mboard will not boot all the way.
See response 4 in this:
If it is failing, you can usually replace it with any decent standard sized standard ATX PS with the same capacity or greater.
Standard (PS/2) power supply size - 86mm high, 150mm wide, 140mm deep, or 3 3/8" h x 5 7/8" w x 5 1/2" d , or very close to that, though the depth can be more or less for some PSs.
Don't buy an el-cheapo PS.
See response 3 in this:
Your power supply must have at least the minimum capacity required to support a system with the graphics card you are using installed, or the max graphics card you might install in the future.
(Onboard video - video built into the mboard - IS NOT A CARD ! )
If that info is not in the ad for the video card, you can go to the video card maker's web site and look up the specs for the model - often under system requirements - the minimum PS wattage, and, more important, the minimum amperage the PS must supply at 12v is stated. If you don't find that, any card with the same video chipset including any letters after the model number has very similar minimum PS requirements. Some power supplies have two or more +12v ratings - in that case, add those ratings to determine the total +12v current capacity.
If you're a gamer...
In most if not all cases, the max capacity rating of the PS is an intermittent rating. It's recommended that you do not load your PS to any more that 80% of that rating if you are going to be using something that puts a constant load on it, such as playing a recent game for hours on end. In that case, you multiply the min capacity stated for the system with the particular video chipset on the card by 1.25 to find the min. capacity of the PS you should get.