Boot Into GRUB menu

The first step is to reboot into the GRUB menu. If Ubuntu 18.04 Bionic Beaver is the only operating system installed you need to keep pressing SHIFT for the GRUB menu to show up.

Edit Grub Menu

Using arrows select the Ubuntu menu or the menu you use to boot your system and hit the e key. This will allow you to edit the menu:

Navigate down with the arrow-down key until you can see a line ending with:

ro quiet splash $vt_handoff

Using your keyboard replace the following keywords.FROM:

ro quiet splash $vt_handoff

TO:

rw init=/bin/bash

Once you have made the change press F10 or CTRL + x to perform boot into a single mode.

Reset password

Confirm that your system partition is mounted as read/write (rw):

# mount | grep -w /

If all good, reset your root password with the passwd command:

# passwd

Alternatively, you can reset your user password. The following linux command will reset the password for the linuxconfig user:

# passwd linuxconfig

Your root password is now reset. Reboot your system:

# 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 the splash boot option when editing the grub’s menu item.
When trying to reboot with the 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

Src: https://linuxconfig.org/how-to-reset-lost-root-password-on-ubuntu-18-04-bionic-beaver-linux