HP a405n

If it still doesn't work you need to look at the M'Board CPU or RAM as the most likely problem. Though if the PS is OK the M'Board is the most likely culprit.

The HP Units have marginal PS's at the best of times and they can fail without warning. When I used to do Warranty Work for HP I continually ran into problems with PS's that although new just didn't produce enough power to run or start a computer so I wouldn't be trusting the HP supplied PS as working without testing by actually making sure that it can actually power a computer.

Col

Assuming the PSU is good, the MOBO has to do the switching part.

As long as the power switch is connected properly to the MOBO and there are no obvious shorts (mobo-to-case, or shorted out hard drive), then the motherboard is dead. inspect the power supply connector carefully for bent or pushed-in pins.

The best test is to take the mobo out of the case, unplug everything except the processor, and short the power-on pins with a screwdriver. Also clear the bios with the appropriate jumper and/or pull the CMOS battery. IF it wont POST then, it's a goner.

If you had to do component-level repair on the mobo, look for caps, diodes, and surface mount fuses in the voltage regulation section of the mobo. Many mobos have a status LED to tell you if the power is getting to the board at all.