The watchdog indicates that there was a problem detected that required the spa to shut itself down for saftey reasons. Could be a variety of issues. In your case it's probably just the way the power went off.
Like Tony said, if you powered down and back up, and the tub went back to operations as usual, you should be good to go. Maybe keep an eye on things for a week or so, just to make sure it doesn't kick off again.