"Then he gazed down at the wise old face and tried to absorb the enormous and incomprehensible truth: that never again would Dumbledore speak to him, never again could he help--" - HBP28
The Tower predicts a sudden, dramatic change in fortune or occurrences. It might be a burst in pent-up emotions or a sharp surprise in an otherwise uneventful life. This event may be a harsh emergency, but it's also a piercing awakening as a call to action or insight.
The events at the end of the sixth book throws Harry's world into disarray - it destroys his trust for Snape's allegiance, as well as removing the one security he could always rely on. For most of Harry's life, Albus Dumbledore has always been the all-powerful protector who had the solution to any danger, the answer for any question. With the death of Dumbledore, Harry's final crutch is removed, and he is left truly on his own to come into maturity and face up to the challenges in his life.