Lemon Tree Dropped all it's Young Fruit a few months Ago

young tree or old tree?
if young, they do that. They aren’t grown up enough to carry fruit

Sometimes citrus trees require heavy pruning. If you have many branch off shoots with one limb covered in blooms the tree exhausts its energy thus resulting in dropping fruit or low yield. Pruning the upward shoots and shoots coming off trunk will allow the citrus to provide its energy more equitably.

A Greek gentlemen taught me this technique and showed me how to prune properly. All my citrus now fruits annually with high yields and no loss. You have to prune it while in flower bloom. TIFWIW.

normal and to be expected