There are sporadic reports of people losing their GUI through unauthorized access to the system, so a hard reboot as the first solution might make things worse, depending on the nature of the problem.
I’d start with logging in at the console (assuming it’s local) or with SSH (if it’s under someone else’s control) and checking out the health of the system. Check for things like the existence of the web document root directory and the files contained therein.
While you’re in there, you can also search through the logs and look for unusual activity (long distance calls or remote access attempts that are out of the ordinary).
Once you are reasonably sure that the system isn’t compromised, you can try a reboot. If it comes up, you still need to figure out what the problem was.
If the system is compromised, you will need to format the hard drive and rebuild the server from your backups. If the config is small, you can skip the backups and rebuild by hand.
The ‘yum update’ and ‘fwconsole ma upgradeall’ are both good suggestions after you get the machine back on-line. There have been several hacks that can cause the system to be rendered nearly inop over the past few months, so staying up-to-date is always a good idea.