ATmega16U2 is not working My arduino is A Ok but can communicate through USB

is it me or the ATmega16U2 chip just malfunction on arduino mega 2560 boards. Mainly I use them for 3D printing but from time to time and for no obvious reason I just lose my communication with my board but the atmel 2560 is just working fine. a good question is there a solution for those boards at my desk to be back to life ?

You may be able to re-flash the firmware on the ATmega16U2 using the ISP pins right behind the USB port. I recommend doing that with another Arduino as an ISP programmer.