The expression refers to Jerusalem (personified) that has sat down and so remains sitting on the ground, a posture often adopted by a grieving person (see also Lamentations 2:10; Lamentations 3:16; Genesis 23:2; 2 Samuel 12:16; 2 Samuel 13:31; Ezekiel 26:16; Esther 4:3). However, more is implied with the reference to being lonely.
To sit or dwell alone is used in Leviticus 13:46 of a leper who is ostracized from the community. In the same manner Jerusalem is ostracized from the community of nations. It remains in a state of desolation.1 Furthermore, this expression is a token of deep sorrow (Nehemiah 1:4).2
1 How lonely sits the city that was full of people! How like a widow has she become, she who was great among the nations! She who was a princess among the provinces has become a slave.