I have an older Gateway AMD Athlon 1.2, I am also having the same problem. Recently my PC began to shut dow on it's own with no warning. The first couple of times, I could reboot with no problem. Then I started to have to unplug/plug the power cord for the PC to power-up. . I tried most if not all the solutions on this original post with no LUCK. Now the power BUTTON lights up, after a few seconds the PSU fan begins to spin, then the CPU fan kicks in, but that is as far as it goes. The monitor does not come out of standby when I power the PC, I have a 250 Watt ATX PSU, anyone that may have a possible solution please help. I am trying to rule out any other possibilities before I swap out the PSU for a new oneAny beeping on boot up...does pass post!......have you reset the BIOS ??have you add a new hard ware? does it boot to SAFE mode .....are there any keys on the KEYBOARD to wake up pc.......Questions!!What have you done prior to this problem happening?The update is to your problem....