I am using this as a flight controller for my hexcopter. It is connected to an Intel NUC which is also mounted on the drone. Everything is powered through LiPo batteries with appropriate voltages. The NUC is connected to the PixHawk through the USB connection. The PixHawk also receives power through the LiPo (I think it switches internally to the best source, we need the USB connection for ROS, as telemetry is connected a separate base station computer which has RTK)
On one fine day when we were taking the drone out for a test, the PixHawk powers on, but does not give the usual beep sounds which it used to, it also does not get detected on the USB ports, nor on QGroundControl. The sensors connected to the PixHawk(GPS, etc) receive power and are working as normal.