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When you first turn on your computer, you should briefly see the GRUB boot menu. Choose the Rescue Mode option. Once in Rescue mode use the
passwd command to set a user's password, for example I would type:
Linux is case-sensitive, so make sure you don't have your caps lock on. Also know the default keyboard for #! is Great Britain, so watch out with special characters. When in doubt try a simple lower-case letter password for troubleshooting purposes.