The other day I was testing some circuits around with Uno.
Basically, I was using an external power source of 12V for my circuit and the USB connection for the arduino. Somehow I think I short-circuited, probably by connecting the +12V to the arduino ground. The computer even turned off.
Thing is, now the board isn't detected when I connect it to the PC. It still works as I intended it to work, functioning as a timer, but I cannot access it from the computer. Already tried another USB cable and on another computer, the board simply isn't detected.
Gigabyte:
I already tried the loop back, but there is no connection at all, so no use
It confirms you lost the ATmega8u2. You can probably use FTDI (or similar USB to Serial) cable to program the ATmega328. Or you can try to replace the 8u2, but that's a pretty delicate operation.