I did the quest to repair it a long time ago. I did the option where I extracted tree sap to get it. Months later when I travel to Whiterun, its suddenly gone. How do I spawn it back?

i downloaded the mod which fixes the cities after the Civil war, does that have anything to do with it?

If you're running the unofficial patch check http://www.gamesas.com/topic/1446254-relzwipz-unofficial-skyrim-patch-thread-28/page__view__findpost__p__22310777