Reboot to Grub Menu
In the first step you need to reboot your Ubuntu 16.04 Linux box to Grub's menu. If the Ubuntu 16.04 is the only installation available keep pressingSHIFT
after you start your computer until GRUB's menu appears: 
Edit Grub Menu
Once you reboot to Grub menu, select the first menu item or the menu item you normally use to boot your Ubuntu system and presse
to edit:
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Alter boot menu
Once in the Grub's boot menu edit mode use navigation arrows to locate a line starting withlinux
and edit it to include read-write mode rw
and init=/bin/bash
. For exampleFROM:
linux /boot/vmlinuz-4-4.0-22-generic root=UUID=43ad24d3-e\ c5b-44ee-a099-a88eb9520989 ro quiet splash $vt_handoffCHANGE TO:
linux /boot/vmlinuz-4-4.0-22-generic root=UUID=43ad24d3-e\ c5b-44ee-a099-a88eb9520989 rw init=/bin/bash

Once ready press
CTRL+x
or F10
to boot. Reseting root's password
If all went well you should now see root shell command line and your root partition should be mounted with read/write flags. To confirm run:root@(none):/# mount | grep -w /

Now we are ready to reset root's password. To do so, simply run
passwd
command with no arguments. When prompted enter your new root password: root@(none):/# passwd

All done. Your root's password is now reset.
Reboot System
Reboot your system using the following linux command:root@(none):/# exec /sbin/init
Troubleshooting
Enter new UNIX password: Retype new UNIX password: passwd: Authentication token manipulation error passwd: password unchangedYour root partition is mounted read-only. Try:
# mount -o remount,rw /to resolve this issue.
[ end Kernel panic - not syncing: Attempted to kill init! exit code=0x0007f00Make sure that you removed
splash
boot option when editing grub's menu item. When trying to reboot with
reboot
command I get: Failed to connect to bus: No such file or directory Failed to talk to init daemon.Ignore, and reboot with:
# exec /sbin/init