I'm not really sure what happened.
All of a sudden, none of my applications wouldn't start. However, the only app I had open before I noticed this (Opera) still worked. I logged out and tried to log back in to see if that would solve the problem, and GNOME wouldn't start.
The loading window would pop-up, hang for a minute, and then crash back to the login screen.
I can still start KDE and my other desktops.
Can anyone tell me why this may have happened? And if at all possible, can somone tell me how to fix it?