Typically, when you have no sign of life at all when you hit the power button, it's either the power supply, motherboard, CPU, or a dead short. Since your system as been assembled and running for quite sometime then that likely eliminates the dead short, leaving the power supply, motherboard or CPU as the likely suspects.
Do you have a digital multimeter? If so you can test the PSU to see if that is the problem (which I believe it is).
If you can let us know what country you are in and your budget for a new PSU then we can give a better suggestion on a quality replacement for the FSP PSU that you have now.
