This guide will provide you with an information on how to reset lost root ( administrator ) password on Ubuntu 16.04. This guide assumes that you have the actual physical access to your Ubuntu 16.04 Linux box.

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 pressing SHIFT after you start your computer until GRUB's menu appears: reboot your Ubuntu 16.04 Linux box in Grub's menu

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 press e to edit:
Edit ubuntu's grub menu

Alter boot menu

Once in the Grub's boot menu edit mode use navigation arrows to locate a line starting with linux and edit it to include read-write mode rw and init=/bin/bash. For example
FROM:
linux     /boot/vmlinuz-4-4.0-22-generic root=UUID=43ad24d3-e\
c5b-44ee-a099-a88eb9520989 ro  quiet splash $vt_handoff
CHANGE TO:
linux     /boot/vmlinuz-4-4.0-22-generic root=UUID=43ad24d3-e\
c5b-44ee-a099-a88eb9520989 rw init=/bin/bash
Edit ubuntu's grub item to 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 /
Edit ubuntu
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
password reset with passwd
All done. Your root's password is now reset.

Reboot System

Reboot your system using the following command:
root@(none):/# exec /sbin/init

Troubleshooting

Enter new UNIX password:
Retype new UNIX password:
passwd: Authentication token manipulation error
passwd: password unchanged
Your 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=0x0007f00
Make 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