I'm pretty sure I fried my Arduino Uno. Given my setup, what caused it to be fried? (I can't identify the short)
The Uno is not one "thing" that fails. It has various components, some of which may have failed. I wrote a lengthy page about how to test various parts:

Have I bricked my Arduino Uno? Problems with uploading to board




  I plugged my 12V 8A power supply into the power input on the arduino. 


Perhaps you reversed the leads, and shorted the USB power to ground, thus causing the PC to turn off? This would probably also blow most of the components on the Uno because they would have had -12V applied to them, assuming it got past the voltage regulator.

However there is a reverse polarity protection diode on the power input jack. Did you use that or the Vin pin? (or the Vcc pin?)

Note that the reverse polarity protection does not protect you if you use the Vin pin.