Well I once applied -12vdc to an analog input pin on a breadboard mounted 644p chip. Saw a quick flash and heard a sharp quick crack sound. However the chip continued to function minus that pin and the next three adjacent analog input pins. So it's not always easy to say just what damage such a 'brainfart' might do. However if the PC does not see the the USB connection when plugging the board in that means the USB serial converter chip went to go where chips go to die most likely.
Lefty