You can reconfigure the xserver from the rescue mode on grub or the
commandline. I would check that the drivers are installed first using
hardware drivers. It is likely that you updated the kernel as part of an
upgrade and the kernel and xserver are mismatched. This sometimes happens
when an upgrade is partial. See this:
www.cyberciti.biz/.../ubuntu-linux-how-to-reconfigure-x-windows-system-xo\
rg-server/

on how to use the commandline to fix the xserver. This is the last resort.
Check hardware drivers first, though.

You will likely have to boot the computer into rescue mode or you can try to
reboot from grub in a previous kernel (lower in the list). You can also try
a boot parameter to run in vesa and then fix it with the proper driver and
xserver. See: https://help.ubuntu.com/community/LiveCDBootOptions