NullPointerException PreparedStatement

Exception in thread "main" java.lang.NullPointerException

at dao.StoreDao.query(StoreDao.java:64)

(64)//PreparedStatement ptmt = conn.prepareStatement(sb.toString());

at test.TestAction.main(TestAction.java:33)

(33)//List<StoreMoudle> gs = g.query();

conn代码//public static Connection getConnection(){
   return conn;
}