You will get chance to appear in the next semester exam. But you have to clear that backlog subject in 3rd sem or in any odd sem. If you dont clear that backlog, you will not get the degree B.Tech after 4th year. Then you have to continue your cource for more year(s)until you clear that backlog. As you asked,there will be no prblm in promotion,but you have to clear that subject to get the degree.