Boot up the machine, and after the BIOS screen, hold down the left Shift key. You will then be prompted by a menu that looks something like this:
I've noticed on some systems that timing when to hit the left Shift key can be tricky, sometimes I miss it and need to try it again.
Hit the down arrow until you select the 2nd entry from the top (the one with the recovery mode in the description) and then hit Enter.
Now you should see this menu:
Using the arrow keys scroll down to root and then hit Enter.
You should now see a root prompt, something like this:
root@ubuntu:~#
At this stage you should have a read-only filesystem. You have to remount it with write permissions:
mount -o remount,rw /
Now we can set the user's password with the passwd command. (In this example I will use jorge as the example, you need to substitute whatever the user's username is):
root@ubuntu:~# passwd techadmin
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
root@ubuntu:~#Alternate Root Shell Method
If you don’t have the recovery mode option, this is the alternate way to manually edit the grub options to allow for a root shell.
First you’ll want to make sure to choose the regular boot kernel that you use (typically just the default one), and then use the “e” key to choose to edit that boot option.
Now just hit the down arrow key over to the “kernel” option, and then use the “e” key to switch to edit mode for the kernel option.
You’ll first be presented with a screen that looks very similar to this one:
You’ll want to remove the “ro quiet splash” part with the backspace key, and then add this onto the end:
rw init=/bin/bash
Once you hit enter after adjusting the kernel line, you’ll need to use the 'B' key to choose to boot with that option.
At this point the system should boot up very quickly to a command prompt.
Should you have forgotten to add the'rw' option at the kernel, the root file system is still mounted read-only, so enter the following command:
mount -n -o remount,rw /
You can use the following command to reset your password:
passwd <username>
For example root just use this command:
passwd
After changing your password, use the following commands to reboot your system. (The sync command makes sure to write out data to the disk before rebooting)
sync
reboot –f
In some instances –f parameter is necessary to get the reboot command to work for some reason. You could always hardware reset instead, but make sure to use the sync command first.
And now we are be able to login without any issues.