Save approx. 10% of the juice to start another batch. You should be able to successfully use the beets again for another batch. To do this, add the 10% juice, the beets and another tablespoon of salt. Fill with water and start the process again.
Sometimes you can get another batch (a third one). I generally compost the beets at this time and start over with fresh beets. You can still use the starter liquid, just use fresh beets.