I have been wintering my lilies inside over the winter. In the summer they live in a fountain on my deck. This year, only 1 pot has leaves. did the other one die? Or is it just dormant for now?
I would give it more time and even more sun if that is possible. Water Lily's can be dormant for long periods.
You can also inspect the plant; pull up the pot to look for any signs of green.