The hot tub is probably resting on sand. If the tub is empty and any water gets under it, the sand could shift. Water under it freezing will also heave it up. It is important to keep water in the tub year-round (for the weight).
To re-set it to the base you would need to take it completely out and re-arrange the sand. It will be by trial and error, so it may take several tries.
Let us know how it goes for you.