The solution to the problem is to first turn off auto-update for M&B:W. Then go to the TaleWorlds website and download the installer (currently, it's still 1.134). Also, save it for later in case this happens again. Reinstall the game to the same directory it currently resides in (within the Steam directories); there is no need to uninstall first. Start up the game from Steam and, voila, you're back in business.