OS X: reset password mountain lion

#1 fsck -fy
#2 mount -uw /
#3 launchctl load /System/Library/LaunchDaemons/com.apple.opendirectoryd.plist
#4 passwd username

The password should now change as expected, where you can then reboot and use the admin user account as expected. Rebooting is possible through the command line by by typing:

reboot

from:
http://osxdaily.com/2011/04/25/change-admin-password-mac/