that's ubuntu in a nutshell, heh.
sounds like some graphic driver problem?
some info about that might help.
you have to get into recovery mode, preferably without gui, and post some output:
Code:
lspci
sudo cat /var/log/Xorg.0.log* | grep -iE 'vga|video'
dmesg | grep -iE 'vga|video'
might be necessary to adjust the grepping, that's off the top of my head.