Invalid sign on means the account name doesn't exist with Yahoo. This could be because you typed it in wrong or your account was deleted. It may have been deleted because of inactivity, terms of use violation or some one hacked your account and deleted. At this point you may need to create a new account.
This could be several differnt things. The user name is caps sensitive. Make sure you are using lower case or capitals as is needed. The same is true for your password. Make sure you type it in exactly as created. Failing that you may have forgotten your password or the yahoo. com site itself may be down temporarily.
Either you have mistyped your logon, password or both, or your Yahoo account has expired and you did not know it. If it has been a while since you last logged in, that might be what happened.
Invalid sign-on errors generally mean you're either entering the wrong user name or the wrong password. Follow the link below the sign-in box that says "I can't access my account. "
