Check to see if the lines are shorted together with an ohmmeter. Then jumper one end and go to the other and check for continuity. This will tell you what kind of problem you are having.

If a staple went through, I would expect a short across the pair, not an open in one or both lines. It could cut through both conductors, shorting them together. But I doubt a staple would have cut the wire in half. If the line is open, I supose someone might have cut it with a saw or something. Maybe a finish carpenter cut clear through. But maybe the line was moved around enough while people were working that a connection broke loose. So I'd check the ends.