Yeah, hold the upper left button for 15s to force it off, then press it again for 1s to turn it back on.
Mine once froze in clock-mode, which took me a while to notice.
On the 910XT you can lock the buttons (to prevent accidental button pushes) by pushing Mode and Up simultaneously. That also unlocks them. I discovered this once, accidentally, just before a race. When I pushed the start button, nothing happened. WTF? It's easy to do accidentally when you put your thumb on one side of the watch for leverage as you push the button on the other side.
I suspect I was a little sloppy when I stopped the run timer and hit multiple buttons. Didn't notice the combination to undo it, so had to force off as Kenny describes. Weird that it was buzzing the whole time (2 hours) before I got a chance to tend to it. After rebooting, it came up 4 hours 5 minutes off in time until it locked on gps satellites.