First and easiest is like @Thomas_Lerman said, unplug the controller to make sure there isn’t a rogue power bleed going to the valve.
But, chances are that zone valve has a torn diaphragm inside it that needs to be replaced, or possibly some debris that needs to be cleaned out. Lesser chance, but still a possibility is that the solenoid could be stuck, or a seal failed on it. I just had to replace one in my yard that had a failed plunger.