Comment 31 for bug 581157

There've been a number cases of this particular error. There may be different causes but the ones I have seen are not wubi-specific: sometimes when the kernel is updated the initial ram disk is not generated correctly. You can tell If this is the problem by booting a previous kernel - if it works then it's got nothing to do with a corrupt root.disk or unclean ntfs etc.

Once you've booted from the old kernel, you can rebuild the initrd.img on the latest kernel:
sudo update-initramfs -u -k <latest_kernel_release>

If you only have one kernel, then you have to chroot into it to rebuild the initrd.img which is more complex for Wubi.

Lately, most wubi problems are related to bad grub updates: https://wiki.ubuntu.com/WubiGuide#Warning