Sorry for the inconvenience.
You've stumbled upon a temporary problem with Yahoo! Mail. Usually this problem gets resolved quickly, without you doing a thing. Try pressing the Reload or Refresh button on your browser, or logging out or restart the computer and then back into your Yahoo! Account. Hopefully that will take care of things. If that doesn't fix the problem, try again shortly. Sometimes you can bypass this error by using one of these links. http://usa.m.yahoo.com/p/mail
Or http://uk.m.yahoo.com/p/m.