Microsoft Account Password Suddenly Required

Wake up this morning and wife's PC demands a Microsoft Password in order to operate the computer. I know nothing of Microsoft passwords - never needed one before. The solution: Log on to the Internet to reset the password -- BUT you cannot log on to anything without the password. Research on another computer reveals the cute trick of holding down the shift key while restarting in order to get back to a previous build of Windows - BUT you need to do this as an administrator - which you can do by signing into Windows - FOR WHICH YOU NEED A PASSWORD!