I drained my hot tub this winter, vacked out everything and wiped shell dry. Just opened it up as spring has started, and there is a very large crack on a seat going downward, 8-10 inches long. Can't imagine what caused this? Does anyone know? Tub did not have a crack when full. I live in a very cold climate but all was drained and shut down. Can this be fixed? It's a sovereign hot tub 95.

If its fiberglass or acrylic it can be repaired but will likely show. I think you need to do a complete inspection. If it was cold enough to crack the shell who knows what else is bad.