After the recent server restarts the game wanted me to update it but when I click the button it comes up with this error:
"Something went wrong with a file. We’re not sure what caused it, so please try again or click the code below for more information. Sorry about that!
Error Code: BLZBNTAGT00000840"
I’ve tried scanning and repairing the game installation but it only yields the same message. I really hope I don’t have to reinstall the whole game, I was trying to play tonight. 
Hello there 
I assume you have already checked this article!
If yes, can you please go to Task Manager and look for a process called: WorldofWarcraft Voice Proxy.
If it is there running while WoW is not - highlight it then click the End Task button (you may need to click More Details on Task Manager to see all the running processes). Launch the application again and try updating.
You can also try deleting cached files. Close your Blizzard application. Press Windows key and R and then look up those folders :
%LOCALAPPDATA%
%APPDATA%
%TEMP%
%PROGRAMDATA%
In each of those - delete any Blizzard or BattleNet folders. It wont remove your games - so do not worry Launch application and let it download the folders again. Then try updating!
Can confirm it just worked for me.
Once the BattleNet is working it only losing the location of your games and account info no big deal.
I had the same problem and it was the voice proxy thing causing it. That suggested fix is not listed in the possible fixes under the code, so thank you!