There could be something wrong with the power supply. Even though you say the LED is on, there are actually three supply voltages that have to be present for the PC to come on. Your fan power light circuit could be shorting out one of them. Or it could have damaged one output and the LED is run off a different voltage.
Try unplugging the fan light circuit and see if that helps. If not, try using a different power supply temporarily. If it still doesn't work, unplug all unnecessary components and try again. Good luck.