The KJV translation leaves out the subject in the first phrase that would make the translation clearer to modern English speakers. If we insert "and forgettest thou..." the sentence makes more sense (other translations do have it explicit).
As the Lord moves to his next question, he parallels Israel's evocation of the sleeping Lord with the counter indictment of their forgetfulness of their God. Have they forgotten this powerful Lord? Not only is he the one who led them out of Egypt, but he is the one who has created the heavens and the earth. In Israel's forgetfulness of the power of the Lord, they have become fearful of man. The Lord also accuses them of inventing their fear: "where is the fury of the oppressor?"
While Israel is accusing the Lord of slumber, he reminds them of his constancy, and reminds them also that their fear of man is greater than their respect for the commitment of the Lord. There is no urgency, the Lord cautions. The Lord is counseling for patience.