"What could be wrong with it."

Could be any number of things - most probably something lower down on the trunk, or problem with roots.
Need a good photo where the dying starts, and of trunk and at soil level.
Do you irrigate? Do you feed?

Your tree looks like it has been both starved and dehydrated for a long time. I would soak the soil for a couple of days and then scratch some citrus labeled fertilizer in the soil. When the tree shows some signs of life a well done pruning would be helpful. Al