At Login: enter your regular username, press enter, and then the password when asked for. Once you have done that, type:
Code:
cd /var/log/ <enter>
less Xorg.0.log <enter>
You can scroll in this file with the arrow keys, PgUp and PgDn. You leave this program by pressing 'q'.
Read through the end of this file. There will be indications what is wrong. Pay special attention to lines starting with '(EE)'.
If you need help with those error messages, copy the lines verbatim in your post. Exactly like they appeared on screen.
jlinkels