Then water it and put it back in good light. It should develop new leaves. White or gray patches on the leaves are most likely a fungus. Increase air circulation, decrease humidity and spray with a mild fungicide. This can be caused from too much water or too little water.
Begonias are also susceptible to gas fumes.
Occasionally, they will have a problem with spider mites. But, if they are kept too wet, begonias can develop root rot. Too much humidity during cooler weather can lead to powdery mildew or gray mold. Inc. All rights reserved.