According to Dorium Maldovar in "The Wedding of River Song", "Silence will fall" is slightly misleading. He said that "'Silence must fall' would be a better translation." Dorium also explained that the phrase referred to the Doctor's future: "On the Fields of Trenzalore, at the fall of the Eleventh, when no living creature can speak falsely, or fail to answer, a question will be asked. A question that must never, ever be answered... The Silence are determined the question will never be answered. That the Doctor will never reach Trenzalore." At the end of that episode, Dorium revealed that the question was "The first question. The question that must never be answered, hidden in plain sight. The question you've been running from all your life: 'Doctor who?'"
That future occurred in "The Time of the Doctor", when the Doctor did finally reach Trenzalore.