Sounds like the X server is unable to load. Look at /var/log/Xorg.0.log, the answer is very likely there.
In case you do not know how to access this file here's how:
Use CTRL+ALT+F2 to switch to a console
type less /var/log/Xorg.0.log
use PageUp and PageDown to scroll
If you find errors you do not understand post them here.