Have you changed something in your graphical configuration or installed a new graphic driver ?
If yes, try the following :
-1- Type Ctrl-Alt-F1
-2- Log in as normal user
-3- Restore the previous state of xorg.conf (with nano)
sudo nano /etc/X11/xorg.conf
-4- Save and close nano
-5- Type
startx, then press Enter
-6- Give us the information, especially the errors messages you get from Xorg. They are preceded by "(EE)".
If no, maybe your root partition is full.
What is the output of the following command ?
du -h