Alrighty, let's try a few things:
1. Clean out any dust to the fans
2. Make sure all internal connectors that go to the motherboard, hard drive, etc. are connected, and that it's properly connected to a power source and the switch on the back is set to on.
3. Does it now work?
4. If not, short the green pin on the 24-pin connector with a black pin, using a bent paperclip. (INSTALL PAPERCLIP WHILE POWER IS UNPLUGGED, THEN WHEN POWER IS PLUGGED IN DO NOT TOUCH THE PAPERCLIP!). Use an old hard drive as a load and see if when you apply power to the PSU if it works. What the green does it's the PS_ON and by shorting that to ground it basically thinks it's hooked up to a motherboard and the computer's power button was pressed.
5. Still no luck?
Read this and then let us know
http://hardwareinsights.com/2011/04/ ... ply-works/