Any building housing a nonconforming use which building is damaged or destroyed from any cause may be restored but not enlarged, and the nonconforming use may be reinstated within one year of such damage or destruction. If the restoration of such building is not completed within said one-year period, the nonconforming use of such building shall be deemed to have been discontinued, unless such nonconforming use is carried on without interruption in the undamaged portion of said building.