when I used the command X -configure in root, this was the output:
[root@localhost ~]# X -configure
Fatal server error:
Server is already active for display 0
If this server is no longer running, remove /tmp/.X0-lock
and start again.
Please consult the The X.Org Foundation support
To perform this you must be root in a virtual console, and quit X. So, logout, press keys Ctrl+Alt+F1, login as root with root as user and root password, then remove the file /tmp/.X0-lock, repeat the X -configure command, follow the instructions and if all good, restart gdm : gdm-start, and...
some useful commands:
and if you are in trouble force reboot with the "reboot" command in your tty.
The command "/usr/bin/nvidia-xconfig" belongs to the packages
therefore if in doubt, you might try to reinstall the 3 above.