root authentication failure.

I am trying to run yum to update to the latest versions of the software but when I "su - root" and enter the password I keep getting an authentication failure error. I am confident I am typing in the right password. I also tried running yum under sudo but got an error message stating that my login is not stored in the sudoers file. When I initially tried to log into the root account I got a different message making reference to a file in etc/...... sorry, I didn't copy the file and location of that file. It also said that I had to reboot the system at that time which I did. Now I am getting the authentication failures.