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.