I had the same issue as @ctlaux. I was making tweaks to my keymaps, and all of a sudden it lost the connection.
For me, unplugging and re-plugging in didn’t work. BUT, I was able to work around by:
- Unplugging
- Hold down prog
- Plug in (while still holding prog). The prog key is illuminated red.
- Start the upload in Arduino.
Thanks to @cyrossignol for the suggestion.