A Windows 8 password must be reset by a domain administrator if the PC is on a domain. The usual method of using a local administrator account or password reset disk does not work. The only other alternative for fixing the PC is to re-install Windows 8.
If the domain administrator account cannot be logged into either, then there are alternate recovery methods available for the domain administrator account. This requires physical access to the domain server and a boot disk. 4SYSOPS maintains an article with step by step instructions on recovering a server if the administrator password is lost. It involves booting from the installation DVD to transfer several files from the boot media to the active file system and then running several commands from a command line prompt.