I have seen this exact problem recently. The computer would refuse to turn back on, but it had worked fine the previous day. Like most people here, I assumed it was a bad power supply or bad power switch also. We tried replacing the power supply and that did not work. We disconnected all power leads on the power supply and removed the power cord from the wall outlet. this enabled the computer to be turned on so we left the computer always on until we could find another solution. If it was a bad power switch why would it turn on after unplugging the power supply? it was not the power switch obviously
The problem happened on a asus K7VTA Pro motherboard. The computer had been working fine for several years until it was moved to a second home. it happened several months later in that place. here's the wierd part: the first two places i was talking about were near pullman, WA. When I took the computer to my home in another city (richland WA, 4 hours away from pullman) I tried to see if I could reproduce the problem and could not. This was the exact same computer that was having problems before, nothing had changed but the location. We even left the PC off and was able to turn it back on overnight.
I finally replaced the motherboard with a new one (ECS elitegroup K7SEM ver1.0). This fixed the problem completely. I came to only 1 conclusion: the AC power must be unstable in the other home. the computer was working in that home fine for awhile though so what I beleive is that the unstable AC power may have slowly damaged the motherboard in some way.
knuckles did all of these problems happen in the same home or possibly all in the same general area (city block, street, town) ??