It sounds like you damaged you motherboard. If you plugged it into wrong pins you probably jammed it, putting pressure on the motherboard, which is never good.
Thanks for the reply! I'm pretty sure you're using "jamming" to refer to some electronic situation but if you meant it as in applying strong physical pressure, the pins went in very nicely. ><
The good thing is that the only thing you could have damaged that is expensive is the motherboard.
This brings me great relief!
I'm sorry for being so dense but:
Is there some kind of testing I can do to confirm that it's the motherboard before buying a new one? Is there no chance it's the PSU? True, I'd never put my ear to it before having this issue, but wouldn't there be some sort of sound from the PSU if it's working correctly? (i.e., fan spinning up)
Thank you so much!