Do you mean that it's literally doing 0 damage, or its not dealing any [i]good[/i] damage for the area you're in? If it's actually doing 0 damage, then you need a new weapon. I might be wrong, but I don't think you can repair dragon weapons without repair powder. Unless you have it, find a new weapon and upgrade it at Andre. If it's not doing any good damage, I'd guess that you're in Anor Londo. The weapon has no scaling with any stats, so it's damage doesn't increase as you level up. By time your around Anor Londo, it's simply outclassed by other weapons. Again, find another weapon.