Certified GKH Gardening Expert
Plant leaves can curl for many reasons. Depending on the plant, it could be natural, but this will depend on the species.
Many plants will have leaf curl due to pH issues, too much, or too little humidity, nutrient deficiencies or toxicity, and many other reasons. This means that it is very important to have pictures and a detailed description of the plant's care, and all information about the climate of the area directly around the plant.
Please include more information and I will be more than glad to help.