You could try changing the boot options in grub so that it boots to a text-only interface, that should fix your problem;
edit /etc/default/grub
sudo nano /etc/default/grub
change the line
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
to
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash text"
sudo update-grub
reboot.
Then when you reboot, the GUI shouldn't start up. If you need the GUI, type startx
to initiate an X Desktop session. If that doesn't work, try sudo service gdm start