Thanks in advance for any help. Ubuntu 12.04

Up until recently my mother has been using the Guest Session when she logs on; now, the Guest Session will not login. When I try to login to it, it bounces me back to the login screen. I then tried to login on a Standard account (Alberta) which I made prior to not being able to login to Guest, and it turns out I cannot login - it gives the "Invalid Password" error. So then I tried to change her password from my own account (Rory) which is the master account. Under the Login Options where the Password option is, it says "Account disabled" and it will not let me change this; I try to apply a password, get no error, but it just still says "Account disabled".

Then I tried to delete the Alberta account altogether, but got this error:

I also tried sudo userdel -r Alberta but got this error: "userdel: cannot lock /etc/shadow; try again later."

I don't know what to do now. Any help appreciated.