Meyer lemon tree leaf drop
It is tricky to grow citrus indoors in pots. They require so much warmth and light to do well, and they really resent being moved into our homes in the winter, which they often show with leaf drop.
Our entomologist looked at your photo and couldn't define pests if present. We would need additional photos of the whole plant and the clear undersides of leaves. That is where you might glimpse scale insects which is the most common problem. Any signs of a sticky honeydew substance?
She thought, in absence of this pest, that it's a matter of nursing it along until it can get more light and go outside again.
In the meantime, try to provide at least 4 hours a day of direct sunlight, and increase humidity by standing it on a tray of moist pebbles and spray mist occasionally. During this rest period, only water enough to keep the soil mix from totally drying out. Don't feed during the rest period.