We have our tree in the house for the winter. Temp stays around 70. We use a grow light about ten hours a day. It has been losing all its leaves.
Myer Lemon Tree
The tree is in shock. Loosing leaves is typical when a lemon tree's growing conditions have dramatically changed. Provided that all of the lemon tree's needs are being met, the tree will recover. GardeningKnowHow gives a few tips on lemon tree care: https://www.gardeningknowhow.com/edible/fruits/lemons/growing-lemon-trees-containers.htm