The PC wouldn't run but it's unlikely to damage the motherboard and correcting this should get it all working again. I've not tried this so I can't say for sure but that's my 'considered judgement'. The power switch only connects for a fraction of a second when you push it and does not have its own power supply. When you reconnect them correctly (ALWAYS use the motherboard manual for reference), make sure that you get the polarity of the HDD LED wires around the right way or it won't light up (if you get the LED polarity wrong though, again a quick swap of the wires should see things working again and when the HDD/Optical drive is busy, the light should flash on).
If you've double checked all your connections to the motherboard and you're having problems, tell us the full details (including all the main system parts and how the problem started).