not Recognize my mega board on port

Hi , problem : at yesterday i use my mega 2560 arduino board and suddenly don't recognize my boar with mac book and power led and led 13 is on but use external led not working and show off

test : i think a problem for wire and change the wire but not working again :'( i think board is wrong and check it , don't see a problem for hardware but a little black point on usb solder ,i think not for this . i thing a problem for power and check this,at 5v ,3.3v ,ground have voltage

using hardware : arduino mega 2560 ,macbook pro

plz help me :'(

and arduino say to me :

Arduino: 1.6.5 (Mac OS X), Board: "Arduino Mega or Mega 2560, ATmega2560 (Mega 2560)"

avrdude: usbdev_open(): did not find any USB device "usb" Error while burning bootloader.

This report would have more information with "Show verbose output during compilation" enabled in File > Preferences.

amirslz74: avrdude: usbdev_open(): did not find any USB device "usb" Error while burning bootloader.

That's normal, unless you are using an external programmer to reflash the bootloader.

Open "System Information" from Spotlight. Look at USB section, paste results here.

I test by other os and other laptop ,at all ,it's same result

Sounds like a ruined 16u2.

You might try these instructions: https://www.arduino.cc/en/Hacking/DFUProgramming8U2

tnx for help