Mine started to drop in the number of steps counted when I walk to and from work each day, was doing about 10000~11000 steps a day, only scraping about 6~7000 steps up until yesterday.
Did the thing that most people did, which was to perform a reset and then a more thorough factory reset.
Now the device sensors are not working at all, not the steps counter, heart rate scanner or even the accelerometers that usually detect your arm lifting up as to display your current stats.
I think an update came through within the last few days ~ a week ago, so suspect that... but also wary it could just that a key component is fried.
Going for an RMA, but maybe someone has come across the same problem and a solution?