Nehemiah’s next phrase explains what he means: how Jerusalem lies in ruins with its gates burned.
The Babylonian army had razed the city in 587 BC, 142 years before Nehemiah uttered these words. In those intervening years, very little had been done to repair those walls and gates (Ezra 4:21). The result is that the Jews now living in and around the city have no protection from (potential) enemies.
17 Then I said to them, “You see the trouble we are in, how Jerusalem lies in ruins with its gates burned. Come, let us build the wall of Jerusalem, that we may no longer suffer derision.”