Boot from the ubuntu CD. Make sure you are indeed booting from the Ubuntu CD. Most manufacturer PC will let you hit F12 on boot to select which boot device you want to use. If F12 doesn't work then change the boot order in your BIOS to boot from CD first.
Once you get the Live CD up and running open a command prompt and run e2fsck on your hard drive with this command.
e2fsck -fpvy /dev/sda5
e2fsck is used to check ext3 and ext 4 File systems. Here is what the options do
-f force check even if FS is marked clean
-p Repair any issues
-v Verbose (Print what it is doing)
-y Assume Yes to all questions