If you have an error code then try to google the solution using it, otherwise, I guess, you've got to reinstall WoW.
You may try to delete "cache" folders, but I don't believe it helps you.
If you don't want to download the whole game, you're able to start playing just using PandaWoW archive. Just unzip it and run PandaWoW.exe, in that case the game will be downloading dynamically, while you are playing. Don't delete your current WoW before you're 100% sure you're able to play with new one.
Regards :)