It's all about grounding!
One of the most common issues that people complain about when requesting help or service is what is nominally known as "total system failure." In this case, the system does not respond when the power button is pressed.
Many times the problem is that the computer does not have proper power applied or that the power button is not securely connected to the motherboard or does not emit a signal when pushed.
However, I have found that grounding is an increasingly large problem - many times, the PC will not turn on because the motherboard is touching the chassis of the case.
If the PC is not assembled well, even a small nudge can make the motherboard touch the chassis. So, before you dismiss any total system failure issue as "motherboard failure," make sure that it is *not* a grounding issue! So many techs without experience diagnose motherboard failure when it is simply a grounding issue.