One of my colleagues today could not work out why he could not draw vertically on the screen. All that would happen is the lines would draw horizontally. First I though it was a glitch and restarted AutoCAD but the problem persisted. It was not ORTHO or POLAR problems but actually a UCS problem.
The UCS had been set so that the drawing was being viewed from the side. All lines were being drawn on another plane! A quick, UCS (enter), W (enter) corrected the problem!