If the computer boots but you can’t access a GUI, use Ctrl+Alt+F3 to open a console. From there you can use nano to edit the login manager configuration. If you had GNOME installed, your login manager is probably GDM, and its config should be at /etc/gdm/daemon.conf, according to the manual. If that is the case, it looks like you should erase the username under the entry “AutomaticLogin=”.