A friend of mine asked me to have a look at there computer for them. It stopped working one day for no apparent reason. I suspected a PSU failure - EZCool ATX 400 jsp 400w. There were no lights on the motherboard, chassis or fans nothing at all.
I checked the PSU and it had blown Capacitors (not completely exploded, but rusted on top) I warned them that it might have damaged the Motherboard but they wanted me to try to find a PSU first.
I found a PSU Corsair vx450w that will connect to the 20 pin and the 4 pin 12v connectors. I connected it up and chassis lights come on and fans spin but no video!.
I disconnected all HDD and CDrom etc so that it was just the 20 pin connector and the 4 pin 12v connector plugged in the monitor to the onboard vga and connected a USB keyboard to see if the lights come on when powered and nothing jut the chassis lights (which has a fancy LED screen with the time, HDD activity, Power status and temperature displayed)and fans spinning not overly spinning either.