Lamentations 5:15–16 (ESV) … “The joy of our hearts has ceased; our dancing has been turned to mourning. The crown has fallen from our head; woe to us, for we have sinned!”
5:15 / Perhaps it was the reference to music making in the previous verse that gives rise to the next thought. Because of their suffering, the joy of life is gone. Dancing, a bodily expression of joy, is also gone, replaced by mourning. Combining the reference to music in verse 14 and the disappearance of joy in verse 15 reminds one of the sentiments expressed in Ps. 137:1–4.
Psalm 137:1–4 (ESV)
1 By the waters of Babylon,
there we sat down and wept,
when we remembered Zion.
2 On the willows there
we hung up our lyres.
3 For there our captors
required of us songs,
and our tormentors, mirth, saying,
“Sing us one of the songs of Zion!”
4 How shall we sing the Lord’s song
in a foreign land?
Lamentations reminds us that it was also difficult, if not impossible, for the faithful to sing joyful songs in the city that had just been destroyed. The psalms also remind us that God was able to reverse this and bring joy out of suffering.
5:16 / Zion had been enthroned among the nations because of God’s blessing upon it. It was king. But because of its sin, its royal status has been removed. In another more literal sense, the crown has also fallen from the head of God’s people. With the Babylonian defeat of Judah, the last king of Judah, Zedekiah, was deposed, and not replaced.
In summary, a veil of gloom hung over Jerusalem. The joy and revelry that had once been there was replaced by sadness and mourning. The bustling activity of a once-thriving city had given way to desolate ruins inhabited only by wild animals. The crown figuratively represented the glory and majesty that had belonged to Jerusalem. That glory was now gone. It was lost because of sin. The people were faint from hunger, and their eyes were dim from tears (cf. 2:11; 3:48–49). Judah had only herself to blame for her present condition of desolation in which wild jackals (cf. 4:3) prowled.
 Longman, T., III. (2012). Jeremiah, Lamentations. (W. W. Gasque, R. L. Hubbard Jr., & R. K. Johnston, Eds.) (p. 392). Grand Rapids, MI: Baker Books.  Dyer, C. H. (1985). Lamentations. In J. F. Walvoord & R. B. Zuck (Eds.), The Bible Knowledge Commentary: An Exposition of the Scriptures (Vol. 1, pp. 1222–1223). Wheaton, IL: Victor Books.