Hold the button to turn off, then hold the button to turn on.
I seem to have the same problem. How do you turn it on again?
Hold the watch face -- settings --- activity tracker --- ensue it's on....
Mine somehow switched off after rebooting -- not sure why.
My vivoactive 3 has stopped counting steps since yesterday. I've checked the walk widget is on and latest firmware installed.
Weird, I scrolled to activities on the normal face and it was all disabled.
It's not the walk widget. It's the system setting for tracking.