As I understand it (and I come from a Debian background not an Ubuntu one) Ubuntu does not directly use the root account. I believe root defaults to no password and no shell. Therefore, you wont be able to use
su to change to root.
To perform 'root' like activities use the
sudo command instead. Type
man sudo for more information, but essentially it allows you to run commands with root's credentials. (or those of another user)
So if you wanted to use Midnight Commander as root, you don't need to do this;
Code:
$su
Password: *******
$mc
you can just do this instead;
As for your resolution issue, try reconfiguring the xserver. Use the following command;
Code:
$ sudo dpkg-reconfigure xserver-xorg
And try to answer as much as you can about your hardware.
Hope that helps a little.