You can start system in single user mode, e.g. use bash as init, and then try to change password of root and (eventauly) user.
During boot in GRUB press 'e' to edit it, and change (changes in GRUB this way aren't permanent and last only until reboot)
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And continue with booting by pressing Ctrl+x. When system launches bash, simply change root password by: passwd
, and eventauly user password: passwd username
. When password is changed, reboot, either by reboot
or shutdown -r now