How did Daniel learn the dream Nebuchadnezzar had forgotten?

The secret was revealed to Daniel in a night vision. This was not human happenstance but the almighty sovereign God at work, who revealed to Daniel the dream that Nebuchadnezzar had forgotten. Daniel had asked the king for time to show the interpretation.

Les explains that Daniel, a young man, spoke with counsel and wisdom to Ariok, the captain of the king's guard, who had gone forth to slay the wise of Babylon. Daniel asked why the decree was so hasty, and Ariok made the matter known to him. Daniel then went in and desired of the king that he would give him time, and that he would show the king the interpretation. Then the secret was revealed unto Daniel in a night vision. Les emphasizes that this is the almighty sovereign God at work, not human happenstance. God now reveals to Daniel the dream that Nebuchadnezzar had forgotten. Daniel was brought before the king, and when the king asked whether he could make known the dream and its interpretation, Daniel testified that there is a God, the God that Nebuchadnezzar knew nothing of.

Daniel 2:19

Then was the secret revealed unto Daniel in a night vision. Then Daniel blessed the God of heaven.

How did Daniel receive the interpretation of Nebuchadnezzar's dream, and what does that show about God's role?

The secret was revealed to Daniel in a night vision, after he requested time from the king. This demonstrates that the almighty sovereign God was at work, not human happenstance, giving Daniel the forgotten dream and its interpretation. Daniel testified that there is a God whom Nebuchadnezzar knew nothing of.