In Ubuntu by default the first user’s account which is created is an administrative account. If you lost your Administrative Account password you will have somehow to reset the password, and yes it’s possible to do it, but from recovery mode. Note this tutorial can work also for Kubuntu.
Here is how to do it.
To reset the password you will have to boot into recovery mode. Power up the computer, after BIOS screen appears hold down the left Shift key. If you don’t get it from first time try again, hit and hold Shift key earlier or later. You should get into following menu.
Now the following menu will appear, with arrows go down and select root line and press enter.
Now you will get into root prompt, before changing the password you will have to remount filesystem with write permissions as now it is mounted with read only. To do this write following command:
mount -rw -o remount /
Now we can set a new password with command passwd for the user we lost the password. For example our user is called simple: user, write following command replacing user with your username you forgot password:
root@ubuntu:~# passwd user
Enter new UNIX password: **Here you type password you want**
Retype new UNIX password: **Here you type one more time the password**
passwd: password updated successfully
If you typed correct password on both lines and got message: password updated successfully, you can reboot your computer manually or with command sudo reboot Now you will be able to log on to your Administrative account with your new password.