att.yahoo.com not loading, cannot access yahoo mail

att.yahoo.com, our home page, will not load. Various attempts to access yahoo mail have failed - error msg: URL does not exist. This is the msg I receive: Sorry, 400 Bad Request. Your browser sent a request that this server could not understand. Please check the URL for proper spelling and capitalization. If you're having trouble locating a destination on Yahoo!, try visiting the Yahoo! home page or look through a list of Yahoo!'s online services. Also, you may find what you're looking for if you try searching below.