PC w/ new motherboard won't start, neither will old one

[JackalStein]

Lately, I just got myself a new ASUS A7V600-X motherboard with an AMD Athlon XP (~2.6 GHz). After changing to the new motherboard, the computer ran smoothly for an hour or so. But after I left my computer for a while, I came back to find that it has been turned off and it would not start. I even tried switching back to my old motherboard (ASUS CUV4X P3 733 MHz) but this time, not even the LED light would turn on and I can't start my computer even with my old motherboard. My power supply has a max of 250W and I know it sounds like my power supply might have worn out. However, the LED light actually lights up on the A7V600-X, but when I push the power button nothing happens. I know that there have been other posts about the same problem with the same motherboard, but they have been rather vague about how the problem was fixed. Perhaps someone can help me out? Thanks.