Comment 76 for bug 204133

If it is an ntfs corruption issue, you can run chkdsk /r from windows on the drive containing the ubuntu installation and then try to boot again.
You can access your data by booting from a linux live CD / USB, then mounting the appropriate windows partition, then mounting the /ubuntu/root.disk file therein using the -o loop mount option.

e.g. assuming your windows partition is /dev/sda1 (change it as appropriate):

mkdir windowspartition
mkdir wubi
sudo mount /dev/sda1 windowspartition
sudo mount -o loop windowspartition/ubuntu/disks/root.disk wubi
#your files are now under the dir wubi
ls wubi
#to undo everything
sudo umount wubi
sudo umount windowspartition