I've had a Mega2560 R3 board that has been working for years until yesterday.
I attached a motor shield with an external DC supply with two servos and two DC motors. Suddenly, the USB port disappeared. I looked into Tools>Port (using Mac OS X) and there was no longer any USB choice.
The Blink program continues to soldier on, so power is still going through the USB port.
Is there something I can do to reset or repair the USB port, or is this board irreversibly damaged?
There's a good chance you fried the USB chip on the Mega. You could try a different USB cable, on the off chance that the data line in the cable is broken.
I did try other cables, so you may have hit it. Thanks for your help.
If it's only the USB chip that's damaged, you could still upload to your board by connecting an ISP programmer to the ICSP header and doing Sketch > Upload Using Programmer. If you don't own a dedicated ISP programmer, you can use a spare Arduino board as an "Arduino as ISP". That's not so convenient as using the USB cable as usual because you can use the USB cable also to communicate with Serial Monitor.
That's a good workaround.
I've done some further research and found that the USB port is supposed to be protected with a polyfuse, and it can take days for the fuse to return to a low enough resistance to be useful. I'll try again in a few days, but am I just hoping against hope?
No, the polyfuse provides power to everything, not just the USB serial interface chip.
This seems to be classic. Board with 16u2, motor controller, usb connected, everything worked fine, then something happens and the 16u2 blows. This is why I don't like boards based on 16u2 as serial interface... I don't think I've ever seen the same story for non-16u2 based boards.
try to run your pc on maintenance mood or safe mood then restart again and then plugin