Nowadays we call it depression, but Freud was right to notice its similarities to mourning. The mourner grieves someone or something she recognizes she's lost. The melancholic lives as if what she's lost is in some ways still present. The depressed are haunted.

Recommended Readings: Freud, "Mourning and Melancholia"

https://www.amazon.com/Freud-Reader-Sigmund/dp/0393314030