I'm pretty sure I fried my Arduino Uno. Given my setup, what caused it to be fried? (I can't identify the short) I was trying to drive a Nema 17 stepper motor using an arduino uno and an A4988 driver. I had it wired up according to this instructable (http://www.instructables.com/id/Drive-a-Stepper-Motor-with-an-Arduino-and-a-A4988-/) at step 2 without the motor power supply wired to the bread board yet. ALSO, I hadn't wired the sleep to reset yet (I just didn't read directions carefully). The board was connected to my computer via USB and I had uploaded the sketch on the instructable to it. 

Then, because I misunderstood the directions, I plugged my 12V 8A power supply into the power input on the arduino. All of the sudden, my computer turned off. My computer is fine after restarting, but now the arduino isn't recognized at all by the computer when I plug it in. 

Given this information, I am pretty sure that my arduino is fried. However, I can't understand why plugging in the power supply to the power input, which has a voltage regulator, would cause the board to fry.
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.