This is what I would do. Did you actually not stand for 12 hours, or was the watch off your hand and didn't record a stand earlier in the day. If so just add a stand hour in the Health app.
If not...that is a bummer. Time to start over.
This happened to me with my read goal and iOS Book app. Last year I had a 300+ day stretch of reading an hour a day. Only to have it ended by a 59 min day!