after update ubuntu doesn't open

You can retrieve data from the Ubuntu virtual disk wubi uses with the
ext2read tool (http://ext2read.blogspot.com/) - just point it at
c:\ubuntu\disks\root.disk

The only workaround I am aware of for the boot problem you are
experiencing is to boot an Ubuntu CD in live CD mode (Try without
installing), then loop mount the root.disk and manually edit the
grub.cfg file:

To do this you must identify the partition that contains the root.disk (this example assumes /dev/sda1), and then edit grub.cfg as follows:
sudo mkdir /media/win
sudo mount /dev/sda1 /media/win
sudo mount -o loop /media/win/ubuntu/disks/root.disk /mnt
sudo cp /mnt/boot/grub/grub.cfg /mnt/boot/grub/grub.cfg.copy
sudo chmod +w /mnt/boot/grub/grub.cfg
gksu gedit /mnt/boot/grub/grub.cfg

Delete all lines up to (but not including) the first line that starts with "menuentry"
Save, reboot.

Notes: the grub.cfg is not supposed to be edited by hand. It gets
regenerated automatically. The workaround may have to be reapplied
later. I don't know of a permanent fix, but when this happened on
release 10.04.1, running "sudo update-grub" fixed it (this command will
also regenerate the grub.cfg, so if it doesn't fix it, you will need to
reapply the workaround).

PS do not update packages grub-pc and grub-common to prevent future wubi problems.

Thanks a lot for your answer, but my daughter knows nothing about the installation and I live in a different city. The only thing that has changed in her computer is her network card. A hardware problem. I suppose sth doesn't match.
Anyway, unfortunately, she decided to go back at windows.
Again, thanks a lot for your time.

I'm pretty certain this has nothing to do with the network card, and everything to do with the latest grub update that is breaking many wubi installs: I've seen a steady stream of support requests on ubuntuforums.org and here on launchpad since the grub update last week.

I understand that these sorts of failures don't look good. In fairness, Windows sometimes has problems too :) but I think it's important for users to use an OS that works for them and that they feel comfortable with.

Finally you proved to be right. Today my daughter called just to say that, in a miraculous way, ubuntu opened again with no other problems.
As for myself, I'd like to ask you about that: do not update packages grub-pc and grub-common to prevent future wubi problems.
Why is that? When the update reports security update and I chose not to install it, is there a possibility to have a future problem as far as the security concerns?
Again, thanks for your time.
PS (Windows sometimes has problems too)
 You're absolutely right :)

The packages grub-pc and grub-common are never listed as Security updates. They're normally in the second 'Recommended' updates part of the list. The update-manager will always prompt you for these updates, but you can go into synaptic, search for grub- and select both of them, then click on Package, Lock version. This will prevent any further prompts to update in update-manager.

I recommend this now because I've seen too many broken wubi installs. The reason is simply that Wubi is not widely used apart from newcomers to Ubuntu, and frankly it doesn't get the testing coverage it should. The last grub update was actually wubi specific, if you can believe it, but clearly not well tested.

But that's my opinion and I'm not a developer or affiliated with Canonical so you are free to make your own decisions.

PS I'd love to know more about how Ubuntu miraculously started working again. Was there anything your daughter tried to fix it? If we can figure this out it might help others in the same situation. Thanks

Thank you very much for your answer, I see to that matter, so the next time I see a grub package, I'll be prepared.
As far as the miracle with Ubuntu started working again, there is nothing else to add.
She told me that being dissapointed, she started making things at Windows like listening to music online and offline, downloading songs and some applications for more security at windows. She didn't touch Ubuntu as she knows nothing about installation and stuff. One day after that, she thought of trying Ubuntu for a last time and that was it.
That's why I used the word miracle.
Since I found you, could I ask something more?
Would it be a problem if I made a disk defragmentation at windows, since there's the virtual partition?
Please if you feel that I overdid it, feel free to say so and not answer at all. Ι apologize.
Have a nice weekend!

I don't think there is a limit to the questions you can ask. No guarantees you'll get a (good) answer :)

I've never had a problem defragmenting with a root.disk present. I didn't get good results though - I think it excluded the root.disk, maybe due to its size (It lists files that couldn't be defragmented afterwards).

Thank you very much, you're so kind!
I'll do the disk defragmentation at last.
Hope that is the last time I bother you. Thanks for everything!