EWTN Catholic Q&A
Pope Benedict XVI
Question from Michael on 04-20-2005:

What would happen if the Pope did not accept the job?

Answer by Matthew Bunson on 04-21-2005:

In that event, the Cardinals would have resumed voting. They would have been entirely free to elect him again. If he had turned it down again, they would have started voting once more until a pope was chosen. Fortunately, Cardinal Ratzinger accepted and chose to become Pope Benedict XVI.