If the computer doesn't detect it then the 16u2 is dead. So toss the board. Or use the ATMega in a breadboard. Try using the Mega to ICSP it with ArduinoISP.
If the computer doesn't detect it then the 16u2 is dead. So toss the board. Or use the ATMega in a breadboard. Try using the Mega to ICSP it with ArduinoISP.