Ubuntu usually checking file system integrity on the hard disk during boot up to avoid data loss. Sometimes this automatic repair doesn’t work perfectly and you need to repair it manually.
The ubuntu cd will be your best tools to repair error on your file system installed in the computer.
First you need to boot using your ubuntu installation cd and run it directly as live cd. Next step is gaining root access to repair the hard drive since you can’t use fsck directly with your normal user account.
Add a new root password for this system by entering sudo passwd root on the terminal and remember it. Now enter the syntax to repair your file system. sudo fsck -f /dev/sda8 (please find out the exact location of your ubuntu installation). When it ask for password, just enter the new root password you’ve entered before.
The option -f on fsck means force checking even if the file system marked clean.
After finishing file checking and repairing you need to reboot your computer and booting it up using ubuntu installation on your hard disk.