Sounds like it might be your case shorting the motherboard. It's going to be painful but you have to go through a process of elimination. First, your PSU, you can usually buy a voltage reader at a PC store for about $10.
Next plug in the I/O plugs in one at a time and turn on the PC every time.
Don't forget to test your HDD, Optical Drive, SSD and your fans.
It could be the plugs or it could be a pin on the mobo.
It sounds like it might be the fans or I/O since it stopped working after they were plugged in.