How have you been watering it? African violets are very picky about how they are watered. Leaves can curl down as a response to being watered with too cold water. They can also curl down if they are over or under watered. You should apply water when the soil begins to feel dry and strive to keep the soil lightly moist at all times.
Broad mites might also be suspect (which reside on the underside of leaves), especially if the leaves feel somewhat brittle. For more information on broad mites, please visit the following PDF link:
Also, if it is exposed to temperatures below 60 degrees it will also respond by curling its leaves.
Lastly, I recommend a review of african violet care, which is found at the following article – be sure your care is meeting all of the african violet’s requirements, particularly with lighting: