Certain things you can do in a user program can leave the board in a state where J-Link is unable to download a new program even if you reset the board or unplug it, because the responsible program is still in flash and is run quickly after reset.
To recover, press the reset button, wait for the LED to flash green, press reset again quickly, and watch for the LED to repeatedly flash red. You can then download a new program.
This was documented in the original HiFive1 “Getting Started”, but unfortunately not in the RevB “Getting Started” document.