You will receive an e-mail the day before the book is due telling you if the renewal was successful or if you must return your books.
If the loan couldn't be renewed it might be for example because someone else has reserved it. In that case, you must return it to the library at the latest on its due date.
When you have returned the book, you can reserve it again, and you will get it when it is your turn. Read more about borrowing on page Borrow.