I cannot boot ubuntu after an update

After an update (using Update Manager) the system asked me to reboot, but then all I got was a prompt with grub>! (I remember to see a grub-update command issued by the installer...)

I'am using Wubi, and the /ubuntu/disks/boot/ doesn't have any files (vmlinuz... or initrd.img...), only a directory (grub/) that is empty (no grub.cfg, .... empty!).

NOTE: (1) I was able to access my ubuntu files from Windows using [https://wiki.ubuntu.com/WubiGuide#How can I access the Wubi files from Windows?]. (2) I checked the integrity of the disks in windows and ubuntu and i didn't found any errors.

It will take me a lot of time to reinstall the system as it was... what can I do to fix this installation?