Fillings sometimes simply fall out. However usually the cause is developing decay underneath the filling. In both cases the tooth is sensitive, especially to cold temperatures and sweet food. To avoid irritation of the dental nerve, the tooth should be treated as soon as possible. Until then, please avoid irritation of the tooth and chewing of hard foods, due to dange of fracturing. To reduce sensitivity until you can consult a dentist, we recommend closing off the hole with sugar-free chewing gum.