no login screen

When you say "no login screen"... do you mean you just get a blank screen or your system hangs while booting... or do you mean it goes straight to your desktop without asking you to login? If it's the later, you can require that the login screen be displayed by selecting the option to do so which is in System Preferences / Users. You'll see the "Login Options" at the bottom of the accounts window on the left. Use the "Automatic Login" drop down to change your options.