lemon tree leaves drooped and the branch went brown.
The brown slowly continued till all branches went brown Eventually the plant died.
Certified GKH Gardening Expert
Without seeing the damage, and just by description alone- It sounds like overwatering may have led to fungal infection.
Unfortunately, this can have many causes and depending on the environment- whether the tree is indoors, or out, whether it is container or in ground, the temperatures around the tree, feeding and watering habits, and even much more.
This article will help you to care for Lemon Trees: https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm