Q.Young Meyer lemon tree.
Planted early fall in South Carolina Beach area. During freezing temps covered with cardboard box. Today leaves are drooping and falling off. Is it dying?
No, this is normal for cooler climates. Just be aware that it may never bear an edible fruit since it will not not have a long enough season. Although they do lose their leaves in cooler environments, they do survive. So as long as your area remains above 20 degrees, then it will survive.
Here is an article on their care:https://www.gardeningknowhow.com/edible/fruits/lemons/growing-meyer-lemons.htm