sorry need more info than that. check /var/log/Xfree86.log or whatever is the last one modified. that will give you a clue.
Actually, do this,
login as root at console,
follow instructions to setup X
sometimes sax will crash when you test the config. if it does, restart the machine and your x maybe back.....
If this doesn't work, post you Xfree86 log here, or post a link to it somewhere as it will be quite long. you could look for errors that will start [EE] at the start of the line.