I'm having this exact same issue. I installed the HD Vintage Pack, then I did a full calibration. Then today I did a couple of changes in the way I was connecting the cables to my amp and suddenly I wasn't hearing my guitar anymore. When I looked to the screen it was full black. I've tried restarting the unit a couple of times and always got to the black screen, no Line 6 logo. I've also tried connecting it to my PC but Monkey doesn't detect it.
Does anyone know how to get around this?
Thank you