Have you ever noticed that we humans hang our heads when we are sad or feel defeated? I'm not sure why we do that. It is such a universal response that it is likely innate and reflexive. When children have been punished or shamed, they hang their heads. When adults have been humiliated or when the burden of the world is on their shoulders, they hang their heads. Job equated the action with misery and disgrace: "If I am wicked, woe to me! And if I am righteous, I dare not lift up my head. I am sated with disgrace and conscious of my misery" (Job 10.15).
Sometimes we probably SHOULD hang our heads. We sometimes do things that are less than noble or from less than noble motives. If we have treated others with dishonor; if we ourselves have acted ignobly; if we are in the wrong — then bobble-heads we should be!
But thankfully, God can restore our dignity and worth. Psalm 3.3 declares of the Lord, "But You, O LORD, are a shield about me, My glory, and the One who lifts my head."
According the inscription at the beginning of this psalm, David wrote this while fleeing from his son Absalom. Absalom rebelled against David and temporarily drove David from the throne and Jerusalem (2 Samuel 15-19). The psalm begins with a declaration by David that his adversaries abound. But despite their threats, he has the assurance of God past deliverances and the promise of future security. It is in this context of trust that David makes the declaration of trust in v 3. Lifting the head symbolizes victory over one's enemies (Psalm 27.6), and here, the restoration of dignity and place.
If your head is hanging because of things you have done — lift up your head, for there is forgiveness. If your head is hanging because your circumstances weigh heavily upon you — lift up your head, for there is hope. If your head is hanging because you're not sure of your worth to God — lift up your head, for there is assurance.