PC not booting no problem slip on some boots and well boot it.
For your computer to boot it usually needs a source to do it mainly your hard drive. However if the problem is your power supply the bad regulation of power could account for it not booting most of the time. I would say check the connections between the motherboard and the hard drive however since nothing at all is comming up then its not getting the chance to load it up. Have you been in your computers innards prior to the booting issues the constant tinkering could be causing static which computers hate.
I believe that anything else which could cause this has already been meantioned but please report any other symptoms or unusual activity for your computer. That or you have a virus in your hard drive that is interupting your booting, I have already heard several cases of it.
Anything else that you believe may have caused it no matter how small let us know.