Before trying to shutdown, could you enter the command:
Do any other users show up besides yourself?
Does Linux mint use policy kit. I suspect that a policy may have been disabled when you made the autologin change (don't know why it should) and not re-granted when you changed it back.
To check if it does use policy-kit, enter this command as your regular user:
If some policies are shown but the four above aren't I think that granting these policies may fix the problem. First see if they are available:
To change the policies, use the "polkit-gnome-authentication" program.
It has a graphical interface and more importantly will save your changes.
Another area to look is in your configuration for users & groups, or for the power-save configuration.