What growing medium (compost?) Have you grown a plant (even a house plant or vegetable) before? If not it might be over-watered. If you are already familiar with how the compost should feel when it's the right moisture, then second guess is it's too humid in there and the leaves can't transpire enough.

Need more information. How old is the plant, how long has it been where it is, how is it situated, fed, watered. How badly wilted is it? Just drooping? Hanging? Changing colour?

(edit: photo answers half of those)