I've also had this issue occasionally. Not really often, but enough to be annoying. The LCD display is black, but the lights on the controls/buttons are lit, and it still plays media, and can skip tracks etc. Switching the unit on/off doesn't cure it. Turning ignition off, restarting doesn't cure it. Turning off, locking car, reopening and restarting doesn't cure it. But if I leave it for a good while (parked at work, or overnight), it works again next time.
Mine recently was recalled to dealer for the "VP2 Radio Software Update" and I have the impression it happens a bit more often now than it did before that software update.
I suspect this is some kind of software bug. I haven't tried removing the USB stick to see if that helps, but will do so next time it happens.