What then shall I do when God riseth up? - That is, when he rises up to pronounce sentence upon people, or to execute impartial justice. Job admits that if he had done injustice to a servant, he would have reason to dread the divine indignation, and that he could have no excuse. "I tremble," said President Jefferson, speaking of slavery in the United States "when I remember that God is just!" Notes on Virginia.
And when he visiteth - When he comes to inspect human conduct. Umbreit renders it "when he punishes." The word visit is often used in this sense in the Scriptures.

What then shall I do when (k) God riseth up? and when he visiteth, what shall I answer him?
(k) If I had oppressed others, how would I have escaped God's judgment.

What then shall I do when God riseth up?.... That is, if he had despised and rejected the cause of his servants, or had neglected, or refused to do them justice; he signifies he should be at the utmost loss to know what to do, what excuse to make, or what to say in his own defence, when God should rise up to defend the cause of the injured; either in a way of Providence in this life, or at the great day of judgment in the world to come, when everything will be brought to account, and masters and servants must stand alike before the judgment seat of God, to receive for the things they have done, whether good or evil:
and when he visiteth, what shall I answer him? when he makes a visitation among men, either in this world, even in a fatherly way, visits transgressions, and reproves and corrects for them; had he been guilty of ill usage of his servants, he must have silently submitted to such visitations and chastisements, having nothing to say for himself why he should not be thus dealt with; or in the world to come, in the great day of visitation, when God shall make inquisition for sin, and seek it out, and call to an account for it; and should this be produced against him, even contempt of the cause of his servants, he was sensible he could not answer him for it, nor for anyone sin of a thousand, as no man will be able to do; but must be speechless, unless he has a better righteousness than his own to answer for him in that time to come. This is Job's first reason which deterred him from using his servants ill; another follows.

Parenthetical; the reason why Job did not despise the cause of his servants. Translate: What then (had I done so) could I have done, when God arose (to call me to account); and when He visited (came to enquire), what could I have answered Him?