http://askubuntu.com/questions/487868/no-tty-after-installing-nvidia-driver

Open up a terminal in the GUI, and sudo nano /etc/default/grub.
Add either of the following to the GRUB_CMDLINE_LINUX_DEFAULT line:
   nomodeset
and optionally
   vga=xxx
Where xxx is a VESA screen code that best matches your screen resolution. You 
can find a list of these here 
(http://en.wikipedia.org/wiki/VESA_BIOS_Extensions#Linux_video_mode_numbers).
Once you are done, save the file, and run sudo update-grub. Reboot, and the 
TTYs should be working again.

when you use the nvidia driver and grub2's gfxterm, you have to use nomodeset 
otherwise you cannot switch to a tty. vga=xxx is optional to change the resolution.