burns.p, have you checked that the password being entered on the page matches what you are expecting as you type, by enabling any 'show password' option on the sign in form? Sometimes what a user believes they are typing does not match what they are actually typing if, for example, the capslock has been accidentally turned on, or for some reason the keyboard layout that Windows is using has changed. Or maybe a key on the keyboard has become stuck? Worth checking, if you haven't already?