Ramban: Due to the extent of their sins - that they placed their faith in other gods, as the Pasuk goes on to explain, Hashem will not forgive them, and will continue to hide His Face from them, but only with regard to not redeeming them from Galus. ¹

Seforno: In response to their claim that Hashem is not in their midst, Hashem is stressing that He will not remove His Shechinah from them, ² but only hide His Face from them - in order not to save them. ³

Hadar Zekenim: It means that will not listen to his Tefilos.

Divrei Eliyahu: Normally, when Hashem punishes Yisrael, they Daven and break the barriers between them and Hashem. But now Hashem will hide from them the fact that the punishment is because He hid His face, so they will not Daven, and receive their punishment.

Chulin, 139b: "Hastir Astir" is a hint in the Torah to Esther (in the days of Esther, Hashem will hide His face, and Haman will attempt to destroy them - Da'as Zekenim).

Seforno: The reason that Hashem will hide His Face from them is because instead of turning to Him with Teshuvah and Tefilah when they were in trouble, they turned to other gods.

Chagigah, 5b: It implies "ba'Yom", 've'Lo ba'Laylah', ¹ to teach us that the Hester Panim is confined to the day- time, but that in the night, "ba'Chalom Adaber bo" - He will reveal to them the source of the Tzaros, to enable them to Daven for its removal. ²

Ramban: Refer to 31:18:1:1.

Seforno: Refer to 31:18:1.1:1.