1)
What fell?
1.
Ibn Ezra: The Mikdash, the place of the Shechinah.
2.
R. Avigdor: Tzidkiyah was blinded, like the coming verse hints to (refer to 5:17:2:2).
2)
What is the meaning of "Oy Na Lanu Ki Chatanu"?
1.
Ri Kara: The Oy (woe) that we experienced is because we sinned.