This is a common problem with that particular lens. I experienced the same after reassembling my lens, when replacing the aperture flex cable (another common problem). Usually this is due to a single screw inside the lens that is blocking the zoom mechanism, and it is not possible to fix without disassembling the lens - which is not trivial.
There are several posts on different forums regarding this particular problem - and there's even the following video tutorial: https://www.youtube.com/watch?v=qml1ukgm58Q