I just brought my lemon tree inside for the fall/winter. The leaves are dropping off and the green fruit is not turning yellow.
Leaves Dropping Off My Lemon Tree
I had this same problem with my Meyer Lemon Tree. The fruit may stay dormant as a green color all winter as mine did, and then turn yellow in the spring. My tree had absolutely no leaves on it in the beginning of the summer, but it miraculously recovered when we placed it outside. Make sure it has enough sun because lemon trees do not love this New England weather. Maybe try a very small amount of all purpose fertilizer. DO NOT OVERWATER this kills most of my plants.