from what i understand when there's a leak in a valve the pressure drop causes ice to form on the outside near the leak. If the ice gets so thick as to impact the actuator it can get stuck in its current position. The motor/hydraulics or whatever is used to physically turn the valve can't break the ice so it's just stuck.
so someone probably needs to go find the valve and figure out what's leaking and either put in a new one or tighten the bolts a bit more :)