ubuntu 11.04 with windows lost boot

Can you give the output of:

sudo fdisk -l; lsb_release -a

Thanks

sorry to say that i could not understand the technical terms since i am new to UBUNTU.

Have you installed Windows after Ubuntu? Have you installed Ubuntu without WUBI?
If yes, follow this video:
http://www.youtube.com/watch?v=8lod8sRb_6I&feature=youtu.be

see the description for more info.

Commands used in this video:

sudo mount /dev/sda5 /mnt *****Using the linux installation on your hard drive instead of /dev/sda5
sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys /mnt/sys
sudo chroot /mnt
grub-install /dev/sda *****Using your boot device instead of /dev/sda
CTRL-D
sudo umount /mnt/dev
sudo umount /mnt/proc
sudo umount /mnt/sys
sudo umount /mnt

I Have installed Ububtu on Windows XP
I installed Ubuntu with WUBI installer

Ok so I think that this will not work.

What is the output of:

sudo apt-get -y install os-prober; sudo os-prober; sudo update-grub

It is a terminal command.

@Enrico: please link question about Wubi to project "wubi", so ~bcbc will have a look at it.

What happens when you try to boot Ubuntu - do you end up at a grub prompt. Is your virtual disk (\ubuntu\disks\root.disk) still there. If not, see http://ubuntu-with-wubi.blogspot.com/2011/08/missing-rootdisk.html

If you still have the root.disk, post back with some more info about what happens when you try to boot.