Hello there,

You might be able to work around this in this way:

1. Navigate to your World of Warcraft folder
2. Run the Repair program
3. Run the program BackgroundDownloader (as an Administrator on Vista and Windows 7) for around 30 seconds
4. Run the Launcher (as an Administrator)

This may initiate a large download, however after this download has completed the game should work again.

Instead, you can try to reinstall the game. At the moment the quickest way to reinstall would be to use the Cataclysm DVD if available.

Please see if this helps.