Assuming that you didn't short or send a surge through anything, my guesses would be:
1. You loosened a PSU connector either on the motherboard or on a device. I would think it most likely that the connector on a HDD is not connectiing well. The little metal sleeves inside the molexes can get spread open and become loose after removing/replacing. You may have just made a loose one more loose.
2. You changed the way devices were situated on the PSU connectors. You may have one overloaded (especially at the HDD or video card), or you may have put fans on a lead with a device, making it unstable due to signal noise from a fan. Try to keep fans on leads separate from any other type of device.
3. You damaged your power supply by plugging fans in with it running (I doubt it, but who knows). Check the output with a multimeter.