You should be able to login as root from a
tty. Most distros will not allow
root to boot an X session by himself therefore you cannot do it from the GUI.
If I am not mistaken Mint uses
tty 7 for the graphical display, therefore we will use
tty 3. Do the following.
Press Ctrl+Alt+F3 to enter the 3rd
tty (if that do not leave the graphical display try Ctrl+Alt+F4 for the 4th
A login form will appear, login as
chown for the correct user:
chown -R <user>:<group> /home/<user>
After that you should be able to login normally through the graphical display. Press Alt+F1, Alt+F2, and so on until you reach the graphical display.