Technically there are places on earth that are below sea level, so if that's your measure for altitude you could have negative altitude and still be flying. But I don't think you could ever have negative AGL.
Does it work if you unplug and replug? If you hit a button other than power does it wake up, or does it say "no signal" or something? Does the laptop see it? Anything in any log? Can you force a redetection from the laptop?
Assuming there were arrests.