I would start by power cycling the watch. You didn't say which one you own, but typically holding down the power button (could be the start/stop button depending on model) until the screen goes dark, 15 to 30 seconds. Then hold the power button again for 1 second to power it on.
Then there is the possibility that the wrist HR monitor is not turned off. Again, depends on model, but go into settings > Wrist Heart Rate > Status > On
