Misplaced standoff, simple noob mistake.
You only need a standoff for each hole in the mobo.
Sounds like you probably fried something, considering the back of your board was grounding out on metal for god knows how long.
Just pray it didn't damage your other components. Be glad you didn't have a fire at least.
Yeah, I felt really stupid once I realized what was wrong... The computer had shutoff when inserting/removing a USB drive too... That should have alerted me.
Any chance I damaged the processor?