I had that problem a couple times myself over the years with it.
1) First time, I uninstalled, restarted computer, then reinstalled and all was ok.
2) Second time, I just had to remove the current patch it was working on updating, then re-run the updater and it continued alright. Should be a patch folder or something like that was dates on the patches.
I also seem to recall having that issue another time and I just ran the repair tool that is built into the game. It should be in your WoW Game folder.
Not sure if any of that made sense or not, it's been a long time since I've had those issues but those steps for resolving issues are pretty much universal across most MMO platforms.