To be able to do administrative tasks on your computer, your user has to be member of the group sudo, which is normally done by the installer for the first user created. For every administrative task, the system will ask:
- For your password if you (the current user) are member of the
sudogroup. - or for the real root password if you are not member of the
sudo
So, i assume that you have configured a password for your user during installation and also ticked automatic login and now have forgotten your password as you didn’t needed it for some time. If this is the case, you need to reset your password.
Follow the instructions in the thread below to reset your password: