Use the Ubuntu Live CD to mount your local installation

If you’re having trouble booting into Ubuntu, you can boot off the Live CD/DVD, and click on “Try Ubuntu”. Then, fire up a terminal window and type

sudo mount /dev/sdXX /mnt  (XX is where your installation is, e.g. sdb1)
sudo mount --bind /dev  /mnt/dev
sudo mount --bind /dev/pts  /mnt/dev/pts
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys  /mnt/sys
sudo chroot /mnt

These commands will mount your Ubuntu system in /mnt, and the last command will change the root directory to /mnt, so whatever commands you issue will now apply to your original Ubuntu installation (not the Live CD).

After you’ve diagnosed and repaired the problem, type

exit
sudo umount /mnt/dev/pts
sudo umount /mnt/dev
sudo umount /mnt/proc
sudo umount /mnt/sys
sudo umount /mnt