I had this happen to me with my old fenix 3. Happened in the middle of a 70.3 too. screen went black and no response to buttons or rest attempts. It actually kept recording for a while though. I was still able to plug it into the computer and retrieve files off it, but could never get it to turn back on. Looking at the last activity when it died, the temperature ramped up to 55 deg C (max it will read, but it probably went higher) and it must have fried. I warranted it with no issue.