It’s not very common but at some point everyone needs to get back into a Linux box they do not know the password to.
The procedure below is for Ubuntu 14.04 with Grub.
Reboot the Linux machine
When you get to the Grub screen select the “Advanced options for Ubuntu”
Ubuntu Grub screen
Then the “(recovery mode)” boot option.
Ubuntu Grub Recovery Mode
This will Boot into recovery mode.
The next thing you need to do is mount the File system and read/write.
Select the Network option. This will enable read/write on the boot system.
Ubuntu - Recovery mode - Menu
select “Yes” to enable Read/Write
Ubuntu - Recovery mode - Mount Filesystem
This will bring you back to the “Recovery Menu”
Select “Root” this will drop you unto whats called “single user mode”. From here you have a Shell as root.
Ubuntu - Recovery mode - Root shell
Then set you new password using “passwd”
you can also use “passwd -u myuser” to set the password of any user.
Ubuntu - Recovery mode - passwd
Then exit the shell (exit) and select “resume” from the “Recovery menu”
Ubuntu - Recovery mode - resume
Now you can log in as root with the password you set.