Never run X as root. It's possible the root account locked something some how.
as root or use su
remove the .Xauthority and .xerverauth.4729 files if they are in /root (note that is root's home directory. It is not /) Generally, /root is empty because you don't log in as root. However, slackware does not impose a method for you su(do) and that's left up to the user to decide how they want to handle it.
and other online resources will help you set up sudo the way you want it.
You should post future slackware related issues in the slackware forum
http://www.linuxquestions.org/questions/slackware-14/
However, this issue was actually a general linux x question so it worked just as well here.
Good Luck