Can't use or change my system password. Lion 10.7.5
Perhaps try enabling the root user from single user mode?

launchctl load /System/Library/LaunchDaemons/com.apple.opendirectoryd.plist
passwd root


https://www.personalmacgeniuses.com/2013/10/31/case-of-the-missing-admin-account/#.V8qUxGW_2AU