Well, it sounds like something has altered the xorg.conf file i.e. the file that controls how stuff is displayed, with what "kit".
If it won't start the graphic window, does it default back to a command line window (which is usually what happens when the x server doesn't start) ???
Cos if it does, then you can check the /var/log/Xorg.0.log too see if it's returned any errors - often tells you why it won't start and stuff like that. Then someone might be able to direct you as to what you need to do/modify/repair etc etc.
Any errors usually start with (EE), don't worry about (WW) - warnings or (II) information. It's the the (EE) errors you'd need to track down first.
p.s. but if you just want to get back up and running, and haven't created loads of personal data - you could always just try the "newbie cure all", and re-install over the top - usually works.