The basic cause is the plant's inability to move water from its roots to its leaves. This may be because the root ball has dried out and cannot be easily rewetted. It can also mean that the soil is too wet and/or fungal diseases have rotted the roots. Dig up the plant and check the rootball. If it is excessively dry, soak it in water for half an hour or so after teasing apart the roots, then replant. If the rootball is saturated or shows signs of rot, there is, unfortunately, almost nothing you can do to save the plant. You can try raising the plant above ground level and adding a lot of bark to the planting mix. This will not cure root rot, but may help if the problem is poorly drained soil.