Turn you us unto you, O YHWH, and we will be turned,
Renew our days as of old.
Unless (ki 'im) you have utterly rejected us,
You are very angry against us.
But he also realises that they cannot expect YHWH to act if they remain unchanged. There had to be a true turning to God. But he recognises that it will not come just from the people themselves. So he calls on YHWH to right the situation. Let Him turn His people towards Himself, and then they will be turned. He recognises that man's sinful condition is such that unless he is turned by the Lord he will not turn.
Let Him ‘renew their days as of old'. He recognises that what was needed was a complete renewal resulting from repentance and a true response to God. Compare Psalms 51:12; Jeremiah 31:18.
But then he adds a proviso, although he cannot really believe that it can be so. What if YHWH has utterly rejected them? What if He is still very angry with them? Those are the only reasons that he can think of as to why YHWH should not act.
And so the book ends on the note of a plea for true spiritual revival, subject to YHWH's will and purposes. He has removed from despair to hope, a hope based on the salvation of God.