Ideally you'd use a live CD - since you might be able to repair the current Wubi install. If you need data off it you could also try ext2read, which gives read-only access to the root.disk from Windows.
If you decide to access via a new Wubi install, it's critical to copy the
*.disk files outside the
\ubuntu directory before installing, since the first step of the Wubi install is to remove any existing install - and that deletes everything. Normally (on NTFS) there is only the
root.disk that you care about, but if you install on a
FAT32 partition or created a separate virtual disk for
/home you can have others.
Let's say you moved (quicker than copy) the
\ubuntubackup\root.disk. Then when you install a fresh Wubi on the same 'drive', you can mount the old one as follows:
sudo mount -o loop /host/ubuntubackup/root.disk /mnt
Then you can use
nautilus (or some other file browser) to view the files e.g. from a command prompt you could enter:
Wubi problems are often caused by hard reboots, perhaps after an apparent freeze. Forced shutdowns can damage any filesystem, but Wubi is more sensitive since it's whole partition is virtual, the
root.disk. The steps to fix are:
chkdsk /f (or /r) from Windows. The corruption can be on
NTFS and/or on the
ext3/4 filesystem within the
root.disk. In your case, you get past the grub menu, so it's probably only the
ext3/4 filesystem, but I might run
chkdsk check that the
root.disk is still there. This may sound strange, but Windows will sometimes move files it repairs to a hidden
\found.000 directory. It may rename the
chk0000.chk so look for it by size (5-30GB). See this for more info.
fsck on the root.disk. You'll need to boot an Ubuntu CD or USB for this. Then mount the host
NTFS partition. E.g. if your
root.disk is on
sudo mount /dev/sda3 /mnt
sudo fsck /mnt/ubuntu/disks/root.disk
You could try booting an older kernel.
Boot in recovery mode.
Check there's enough free space to boot (by mounting the root.disk from the live CD and then freeing up space if required, or resizing on the fly).