USB not showing in tools>serialport (mac laptop)

i have an arduino uno and the USB option is not showing under the serial port, i already used it before and it was working fine but now that i need it for a school project it is not showing up.

I have the same problem, the arduino is not responding and I am 99% sure that the 16u2 is the problem. I want to write the firmware on it. The difference is that I am using windows. I have installed flip. But the arduino is not seen by the computer. If I connect the reset to gnd nothing happends, the arduino doesn't enter in dfu mode. What can I do to save the board.