Why has the tree (2 years old), which was growing and healthy, now lost its leaves and its branches are going brown?
Certified GKH Gardening Expert
Frost damage, wind damage, phytophthora root rot, bacterial blast, and several other diseases can all cause specific branches on a citrus tree to die and lose their leaves. Please see these articles for more information and links:
Diseases vary by region, so you might want to consult a local extension agent or other expert.