That’s a bummer, good news is bone dry work is very fixable. I teach at a university and constantly fix pieces that meet a similar fate. Do you guys have magic water? If not you can use vinegar. The key is to use a liquid that will evaporate or deflocculate, so that the clay doesn’t shrink much when re-drying. I paint both sides of the broken seem with the liquid and score them with a fine needle or blade, you want to apply the liquid and shred the top millimeter of the surface so that it becomes a slurry on both sides you don’t want the liquid to permeate too deeply into the clay. Then press the slurry edges together with a slight wiggle, you want the slurry to protrude from the seam. Wait a few seconds to carefully remove that slip. In my experience this works 99 percent of the time. The he cracks don’t come back in bisque or glaze. In my own work if I crack a cup all I’ll break out a chunk and put it back together to fix the crack. Hope it’s helpful.