Inability to sign in for two days -- second instance

For the last two days, I've been unable to sign in.  This is the second time it has occurred in exactly the same manner.  When I click "Sign in", it tries and fails to use a cookie to sign me back in (which is not the way it should be by default, as somebody else noted some time ago).  If I clear my cache and cookies, it then presents me with the login screen but still nothing happens.  At that point, however,,. it seems to have stored the cookie, as additional attempts to log in go back like it was before clearing.